Electro-optic displays, and methods for driving same

ABSTRACT

The invention relates to electro-optic displays and methods for driving such displays. The invention provides (i) electrochromic displays with solid charge transport layers; (ii) apparatus and methods for improving the contrast and reducing the cost of electrochromic displays; (iii) apparatus and methods for sealing electrochromic displays from the outside environment and preventing ingress of contaminants into such a display; and (iv) methods for adjusting the driving of electro-optic displays to allow for environmental and operating parameters.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending application Ser. No.10/907,171, filed Mar. 23, 2005 (Publication No. 2005/0152018), whichitself is a divisional of application Ser. No. 10/249,128, filed Mar.18, 2003 (now U.S. Pat. No. 6,950,220, issued Sep. 27, 2005), whichitself claims priority from the following Provisional Applications: (a)Ser. Nos. 60/365,365; 60/365,368; 60/365,369; and 60/365,385, all ofwhich were filed Mar. 18, 2002; (b) Ser. Nos. 60/319,279; 60/319,280;and 60/319,281, all of which were filed May 31, 2002; and (c) Ser. No.60/319,438, filed Jul. 31, 2002. The entire contents of theaforementioned applications are herein incorporated by reference. Theentire contents of all patents and published applications mentionedhereinafter are also incorporated herein by reference.

BACKGROUND OF INVENTION

The present invention relates to electro-optic displays and methods fordriving such displays. Certain aspects of the present invention aredirected especially to electrochromic displays, and more specifically to(i) electrochromic displays with solid charge transport layers; (ii)apparatus and methods for improving the contrast and reducing the costof electrochromic displays; and (iii) apparatus and methods for sealingelectrochromic displays from the outside environment and preventingingress of contaminants into such a display.

Electro-optic displays are used in a wide variety of devices fordisplaying text, still image graphics, and moving pictures. (The term“electro-optic display” is used herein in its conventional meaning inthe art to refer to a display using an electro-optic material havingfirst and second display states differing in at least one opticalproperty, the material being changed from its first to its seconddisplay state by application of an electric field to the material. Theoptical property is typically color perceptible to the human eye, butmay be another optical property, such as optical transmission,reflectance, luminescence or, in the case of displays intended formachine reading, pseudo-color in the sense of a change in reflectance ofelectromagnetic wavelengths outside the visible range.) Devices thatcurrently use electro-optic displays include digital wristwatches,calculators, personal digital assistants (PDA's), flat screen computerdisplays, laptop personal computers, and cellular phones. As theelectro-optic display has evolved into an important and versatileinterface to modern electronic appliances, the microelectronics industryand the display technology industry have formed a powerful technologypartnership in developing new applications. The display industry hascontinually introduced new technologies for improved electro-opticdisplay performance and the microelectronics industry has followed withthe hardware and software to support these new displays.

The first electro-optic displays, including Nixie-tubes, were expensiveand fragile, were severely limited in the data they could present, andrequired a significant amount of power, space, and support electronics.This hampered both their usefulness and acceptance. With the advent ofthe solid-state light emitting diode (LED), the cost, size, and circuitcomplexity for electro-optics were reduced and the technology was widelyaccepted, especially in numeric display applications. Control circuitrywas easily implemented with existing digital circuit techniques butstill required high power drive stages. Since LED's still drew asignificant amount of power and could not easily scale to meet theincreasing market demand for higher resolution displays, displaydevelopment moved to new approaches.

In searching for a better method, investigation focused on opticalproperties of liquid crystals. U.S. Pat. No. 3,932,024, assigned to DaiNippon Toryo Kabushiki Kaisha (Osaka, Japan), describes a liquid crystaldisplay device comprising a pair of opposed electrode-mounted plates,each of which is provided with a polarizer, the planes of the twopolarizers being perpendicular to each other, and a nematic liquidcrystal layer. The electrode terminals are mounted on one plate bytransferring the connection of one electrode to the opposite platewithout directly contacting the liquid crystal material by interposingan electrically conductive material between the corresponding electrodeterminal and the one electrode.

Common-plane-based LCD's are good for simple displays that need to showthe same information over and over again. Watches and microwave timersfall into this category. Although a hexagonal bar shape is the mostcommon form of electrode arrangement in such devices, almost any shapeis possible. Examples of some of the electrode shapes defined inapplications such as inexpensive handheld games include playing cards,aliens, fish, and slot machines.

Passive-matrix LCD's use a simple grid to supply the charge to aparticular pixel on the display. The grids are formed on top and bottomglass layers called substrates. One substrate forms the “columns” andthe other substrate forms the “rows”. The wiring of the column or rowsis made from a transparent conductive material, usually indium-tin oxide(ITO). The rows or columns are connected to integrated circuits thatcontrol when a charge is sent down a particular column or row. Theliquid crystal material is sandwiched between the two glass substrates,and a polarizing film is added to the outer side of each substrate. Toturn on a pixel, the integrated circuit sends a charge to the correctcolumn of one substrate and electrically grounds the associated rowwhere the intersection of the row and the column will determine the“pixel” or cell element to be activated. The row and column intersect atthe designated pixel, and that delivers the voltage to untwist theliquid crystals at that pixel.

The passive-matrix system is simple and cost effective, but it hassignificant drawbacks, notably slow response time and imprecise voltagecontrol. Response time refers to the LCD's ability to create or recreate(refresh) the image displayed. Slow response time is especiallynoticeable in pointer- or mouse-driven graphical user interfaces. Inaddition, imprecise voltage control hinders the passive matrix's abilityto influence only one pixel at a time. When voltage is applied tountwist one pixel, the pixels around it also partially untwist, whichmakes images appear fuzzy. Therefore, each pixel lacks contrast with itsneighboring pixel.

The active-matrix LCD was developed to ameliorate many of thelimitations of the passive-matrix display. In this type of LCD display,the addressing takes place completely behind the liquid crystal film.The front surface of the display is coated with a continuous electrodewhile the rear surface electrode is patterned into individual pixels. Athin film transistor (TFT) acts as a switch for each pixel. The TFT isaddressed by a set of narrow multiplexed electrodes (gate lines andsource lines) running along the gaps between pixels. A pixel isaddressed by applying current to a gate line that switches the TFT onand allows a charge from the source line to flow on to the rearelectrode. This sets up a voltage across the pixel and turns it on. Animage is created similar to the passive display as the addressingcircuitry scans across the matrix. By controlling the amount of voltagesupplied to a pixel crystal, the amount of crystal twist can becontrolled. By doing this in exact, minute increments, active-matrixLCD's can display usable gray scale images. The active-matrix displaytechnology offers improved response time, viewing angle, contrast, andintensity control as compared with passive-matrix LCD's. Hence activematrix displays are the technology of choice for high-resolutionelectro-optic computer applications.

In all LCD displays, the liquid crystal material is activated by adiscrete applied voltage (e.g., 5 volts) and the liquid crystal changesits optical properties in response to that voltage. When the voltage isremoved from the cell, the liquid crystal returns to its original state.The hardware that drives the LCD can consist of simple combinatorialdigital logic. Slightly more complex circuits can be used that take intoaccount knowledge of specific display performance variables, such ascell transition times or ambient temperature response. When differentlevels of twist are required, i.e., gray scale, control of the appliedvoltage level is required, necessitating more complex drive circuitryand knowledge of the optical properties versus voltage curve.

To provide full color in an LCD, each individual pixel is divided intothree sub-pixels, normally red, green, and blue (RGB). Applying colorfilters that only allow certain wavelengths to pass through them whileabsorbing the rest creates these sub-pixels. With a combination of red,blue, and green sub-pixels of various intensities, a pixel can be madeto appear any number of different colors. The number of colors that canbe made by mixing red, green, and blue sub-pixels depends on the numberof distinct gray scale levels (intensities) that can be achieved by thedisplay.

The liquid crystal-based electro-optic display industry has been subjectto the same cost/performance market pressure as the microelectronicsindustry. As a result, LCD's have been following a trend towardsincreased density, improved color depth, faster response time, and lowercost. The hardware and software used to control these voltage-drivendevices are well known, well characterized and relatively simple, thusallowing for ease of hardware/software/display integration, and this hascontributed to widespread adoption of the technology.

However, LCD's do have some inherent drawbacks. Transmissive LCD'srequire backlighting, which draws significant power. Also, the contrast,though much improved over early implementations, is inherently limitedto the background and foreground colors and color differentiation.Reflective LCD's (which essentially place a reflector on the opposedside of the display from the observer) have insufficient contrast formany applications. Typically, LCD's require special packaging to keepthe liquid crystal in a predetermined region in each cell and in theoverall display. Furthermore, since the extent of rotation of the planeof polarization of light by the liquid crystal depends upon thethickness of liquid crystal layer, this thickness must be accuratelymaintained, which renders it very difficult to prepare LCD's on flexiblesubstrates.

One type of non-liquid crystal electro-optic display, namely organiclight emitting diode (OLED) displays, has found favor where low power,high contrast, and fast response times are required. The OLED technologyhas shown great potential but limited commercial viability, due to somesignificant performance issues.

OLED's are usually arranged in an active-matrix arrangement, similar tothat used in LCD's. However, the electrical requirements of the OLEDpixel circuit are significantly different. This circuit must provide aconstant current to the OLED device, but the magnitude of the currentmust be controllable over a range of more than two orders of magnitudein order to allow for high-contrast images. Typically, in active-matrixOLED displays, there are two metal oxide semiconductor field effecttransistor (MOSFET) drivers at each pixel. A voltage is applied for aset period of time to the first transistor, causing it to turn on andconduct, storing a charge on a capacitor. This capacitor then connectsto the gate of a second transistor, and causes the latter to conductcharge to the OLED pixel, a process that continues until another signalis applied to discharge the capacitor. Thus, the OLED emits continuouslyat an intensity defined by the rate of charge flow (current).

The current source is one of the two critical components of the OLEDpixel cell, and its design is set by the actual pixel currentrequirement. This requirement in turn is derived from the targetluminance, the OLED efficiency, the color filter transmission (whenused), the relative and absolute areas of the color sub-pixels, and theduty cycle of the pixel.

The second most important component of an active-matrix OLED pixel cellis the storage element. Even though an OLED is current driven, the mostpractical way to store energy is the capacitor. Fortunately, a MOSFET isa fairly good voltage-to-current converter, when driven properly, sothat, in essence, each pixel is driven by a current driver that iscontrolled externally by an applied voltage.

In many ways, the OLED has the same benefits of external drive circuitsimplicity as the active-matrix LCD. However, many technologicalchallenges lie ahead before OLED's can become a commercially viabledisplay technology; these challenges include short operational life andsusceptibility to moisture, which degrades the displays, and requiresthe use of special hermetically sealed packages. Indeed, some modernOLED's are sealed in “can packages” with desiccants inside the packageto absorb moisture. This type of packaging increases the cost of theoverall displays and severely limits the use of these displays, sincethe hermetically sealed packing is difficult to scale down in thicknessand therefore is not useable for very thin display devices (credit cardtype displays, flexible displays, etc.). It is also difficult to scaleOLED displays to large sizes because of high defect densities and thetechnical difficulties associated with making OLED's in large area formfactors.

Another type of electro-optic display is a particle-basedelectrophoretic display, described for example in U.S. Pat. Nos.3,668,106; 3,756,693; and 3,767,792. Particle-based electrophoreticdisplays make use of one or more types of electrically-charged particlesdispersed in a suspending fluid. Known electrophoretic media can bedivided into two main types, referred to hereinafter for convenience as“single particle” and “dual particle” respectively. A single particlemedium has only a single type of electrophoretic particle suspended in acolored suspending medium, at least one optical characteristic of whichdiffers from that of the particles. (In referring to a single type ofparticle, we do not imply that all particles of the type are absolutelyidentical. For example, provided that all particles of the type possesssubstantially the same optical characteristic and a charge of the samepolarity, considerable variation in parameters such as particle size andelectrophoretic mobility can be tolerated without affecting the utilityof the medium.) When such a medium is placed between a pair ofelectrodes, at least one of which is transparent, depending upon therelative potentials of the two electrodes, the medium can display theoptical characteristic of the particles (when the particles are adjacentthe electrode closer to the observer, hereinafter called the “front”electrode) or the optical characteristic of the suspending medium (whenthe particles are adjacent the electrode remote from the observer,hereinafter called the “rear” electrode, so that the particles arehidden by the colored suspending medium).

A dual particle medium has two different types of particles differing inat least one optical characteristic and a suspending fluid which may beuncolored or colored, but which is typically uncolored. The two types ofparticles differ in electrophoretic mobility; this difference inmobility may be in polarity (this type may hereinafter be referred to asan “opposite charge dual particle” medium) and/or magnitude. When such adual particle medium is placed between the aforementioned pair ofelectrodes, depending upon the relative potentials of the twoelectrodes, the medium can display the optical characteristic of eitherset of particles, although the exact manner in which this is achieveddiffers depending upon whether the difference in mobility is in polarityor only in magnitude. For ease of illustration, consider anelectrophoretic medium in which one type of particles are black and theother type white. If the two types of particles differ in polarity (if,for example, the black particles are positively charged and the whiteparticles negatively charged), the particles will be attracted to thetwo different electrodes, so that if, for example, the front electrodeis negative relative to the rear electrode, the black particles will beattracted to the front electrode and the white particles to the rearelectrode, so that the medium will appear black to the observer.Conversely, if the front electrode is positive relative to the rearelectrode, the white particles will be attracted to the front electrodeand the black particles to the rear electrode, so that the medium willappear white to the observer.

If the two types of particles have charges of the same polarity, butdiffer in electrophoretic mobility (this type of medium may hereinafterto referred to as a “same polarity dual particle” medium), both types ofparticles will be attracted to the same electrode, but one type willreach the electrode before the other, so that the type facing theobserver differs depending upon the electrode to which the particles areattracted. For example suppose the previous illustration is modified sothat both the black and white particles are positively charged, but theblack particles have the higher electrophoretic mobility. If now thefront electrode is negative relative to the rear electrode, both theblack and white particles will be attracted to the front electrode, butthe black particles, because of their higher mobility, will reach itfirst, so that a layer of black particles will coat the front electrodeand the medium will appear black to the observer. Conversely, if thefront electrode is positive relative to the rear electrode, both theblack and white particles will be attracted to the rear electrode, butthe black particles, because of their higher mobility will reach itfirst, so that a layer of black particles will coat the rear electrode,leaving a layer of white particles remote from the rear electrode andfacing the observer, so that the medium will appear white to theobserver: note that this type of dual particle medium requires that thesuspending fluid be sufficiently transparent to allow the layer of whiteparticles remote from the rear electrode to be readily visible to theobserver. Typically, the suspending fluid in such a display is notcolored at all, but some color may be incorporated for the purpose ofcorrecting any undesirable tint in the white particles seentherethrough.

Both single and dual particle electrophoretic displays may be capable ofintermediate gray states having optical characteristics intermediate thetwo extreme optical states already described. (The term “gray state” isused herein in its conventional meaning in the imaging art to refer to astate intermediate two extreme optical states of a pixel of a display,and does not necessarily imply a black-white transition between thesetwo extreme states. For example, several of the patents and publishedapplications referred to below describe electrophoretic displays inwhich the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned, the transition between the two extreme states may notbe a color change at all.)

Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, optical state bistability, and low powerconsumption when compared with liquid crystal displays. (The terms“bistable” and “bistability” are used herein in their conventionalmeaning in the art to refer to displays comprising display elementshaving first and second display states differing in at least one opticalproperty, and such that after any given element has been driven, bymeans of an addressing pulse of finite duration, to assume either itsfirst or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin United States Published Patent Application 2002/0180687 that someparticle-based electro-optic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” will be used herein to cover both bistable and multi-stabledisplays.) Nevertheless, problems with the long-term image quality ofelectrophoretic displays such as those described in the threeaforementioned patents have prevented their widespread usage. Forexample, the dispersed particles used in electrophoretic displays tendto settle, resulting in inadequate service-life for these displays.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a liquid suspension medium, and a capsule wallsurrounding the internal phase. Typically, the capsules are themselvesheld within a polymeric binder to form a coherent layer positionedbetween two electrodes. Encapsulated media of this type are described,for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564;6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;6,521,489; and 6,531,997; and U.S. Patent Applications Publication Nos.2002/0019081; 2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661;2002/0063677; 2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832;2002/0131147; 2002/0145792; 2002/0154382, 2002/0171910; 2002/0180687;2002/0180688; 2002/0185378; 2003/0011560; 2003/0011867; and2003/0025855; and International Applications Publication Nos. WO99/67678; WO 00/05704; WO 00/20922; WO 00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; and WO01/17029.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display in whichthe electrophoretic medium comprises a plurality of discrete droplets ofan electrophoretic fluid and a continuous phase of a polymeric material,and that the discrete droplets of electrophoretic fluid within such apolymer-dispersed electrophoretic display may be regarded as capsules ormicrocapsules even though no discrete capsule membrane is associatedwith each individual droplet; see for example, WO 01/02899, at page 10,lines 6-19. See also the aforementioned 2002/0131147. Accordingly, forpurposes of the present application, such polymer-dispersedelectrophoretic media are regarded as sub-species of encapsulatedelectrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

It should be noted that, although electrophoretic displays are oftenopaque (since the particles substantially block transmission of visiblelight through the display) and operate in a reflective mode,electrophoretic displays can be made to operate in a so-called “shuttermode” in which the particles are arranged to move laterally within thedisplay so that the display has one display state which is substantiallyopaque and one which is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, International Applications Publication No. WO 02/01281, andpublished US Application No. 2002/0075556, both assigned to SipixImaging, Inc.

Unlike the previous discussed display technologies, particle-basedelectrophoretic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Writing/clearing pixels in particle-based electrophoretic displays, orchanging gray scale in such pixels involves switching voltages on andoff or applying opposite voltages to move the appropriate particles intothe desired position. Knowledge of (1) the initial state of the pixel,(2) the time required to move the particles, i.e., the transition time,(3) the time at voltage curve vs. optical properties, and (4) therelaxation time of the pixel is important to provide a high qualityimage on a particle-based electrophoretic display.

Color electrophoretic displays can be implemented using red/green/blueparticles. To accomplish this, either each particle type needs to reactto a different voltage level or each colored particle within a pixelwould require a separate sub-pixel.

Another type of electro-optic display similar to a particle-basedelectrophoretic display is a rotating bichromal member type asdescribed, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782;5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and6,147,791 (although this type of display is often referred to as a“rotating bichromal ball” display, the term “rotating bichromal member”is preferred as more accurate since in some of the patents mentionedabove the rotating members are not spherical). Such a display uses alarge number of small bodies (typically spherical or cylindrical) whichhave two or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface.

Finally, another type of electro-optic display is an electrochromicdisplay; this type of display uses a material which changes at least oneoptical characteristic as electrons are added thereto or removedtherefrom. (Electrochromism is defined as a reversible color change of amaterial caused by the application of an electrical current.) Severaltypes of electrochromic displays are known. According to U.S. Pat. No.6,301,038 (derived from International Application PCT/IE98/00008,Publication No. WO 98/35267), one type of electrochromic display isbased on ion insertion reactions at metal oxide electrodes. To ensurethe desired change in transmittance the required number of ions must beintercalated in the bulk electrode to compensate the accumulated charge.However, use of optically flat metal oxide layers requires bulkintercalation of ions as the surface area in contact with electrolyte isnot significantly larger than the geometric area. As a consequence theswitching times of such a device are typically of the order of tens ofseconds.

Also according to the same U.S. Pat. No. 6,301,038, a second type ofelectrochromic display is based on a transparent conducting substratecoated with a polymer to which is bound a redox chromophore. On applyinga sufficiently negative potential, the electrochromic polymer istypically oxidized from its neutral state to a deeply colored form, thecolor of which depends upon the nature of the polymer. Among theelectrochromic polymers which may be used in this type of display arepolythiophenes, polypyrroles, and polyanilines. To ensure the desiredchange in transmittance a sufficiently thick polymer layer is required,the latter implying the absence of an intimate contact between thetransparent conducting substrate and a significant fraction of the redoxchromophores in the polymer film. As a consequence the switching timesof such a device are, as above, typically of the order of tens ofseconds.

The aforementioned U.S. Pat. No. 6,301,038 describes an electrochromicdisplay in which the active layer (i.e., the layer whose opticalcharacteristics are varied by addition or removal of electrons) is ananoporous-nanocrystalline film comprising a semi-metallic oxide havinga redox chromophore adsorbed thereto. In the related WO 01/27690 (seealso Wood, D., “An Electrochromic Renaissance?”, Information Display,18(3), 24 (March 2002, hereinafter referred to as the “Wood article”—theentire disclosure of this article is herein incorporated by reference)broadens the concept to a nano-porous, nano-crystalline film comprisinga conducting metal oxide having an electroactive compound which iseither a p-type or n-type redox promoter or p-type or n-type redoxchromophore adsorbed thereto. Furthermore, the present invention furtherbroadens the concept by removing the limitation to adsorption of theelectroactive compound, and provides that this compound may bechemically bonded to the metal oxide. The present invention extends todisplays using this type of chemically-bonded electroactive compound, aswell as to displays using solid electrolytes, as described in moredetail below. Although most aspects of the present invention willprimarily be described with specific reference to embodiments of thetype described in the aforementioned U.S. Pat. No. 6,301,038, thenecessary modifications to embodiments of the types described in theaforementioned WO 01/27690 and Wood article and the types using solidelectrolytes will readily be apparent to those skilled in the technologyof electrochromic displays.

For an electronic display, the electrochromic effect is only useful ifthe color change is truly reversible. Typically, a current flow in onedirection causes a color to form, while reversing the current flowcauses the color to disappear (bleach). Materials showing this effectare known as electrochromic and may be organic (carbon-based) orinorganic in character.

FIG. 1 of the accompanying drawings shows a schematic cross-sectionthrough a display (generally designated 100) described in theaforementioned U.S. Pat. No. 6,301,038. The display 100 comprises afirst glass substrate 102, a first fluorine-doped tin-oxide coatedconductive layer 104, a nano-structured film 106 of titania (TiO₂)coated on the first conductive layer 104, a redox chromophore 108adsorbed on the titania in the film 106, an electrolyte or electrondonor solution 110, a second conductive layer 112 of fluorine-doped tinoxide, and a second glass substrate 114. The redox chromophore can beany of a variety of N,N-disubstituted derivatives of 4,4′-bipyridyl, thepreferred one being N,N′-bis(2-phosphonoethyl)-4,4′-bipyridiniumdichloride (referred to as bis(2-phosphonoethyl)-4,4′-bipyridiniumdichloride in the aforementioned U.S. Pat. No. 6,301,038).

The aforementioned Wood article describes what is apparently a latervariation of the same process, in which the substrates 102 and 114 areformed of glass coated with indium tin oxide (ITO), as used in LCD's.The first substrate 102 (which forms the front electrode in the finaldisplay) is coated with the same anatase titania/redox chromophorelayer, the redox chromophore being a viologen. The second substrate 114is covered (apparently over the ITO layer thereon) with anano-structured antimony-doped tin oxide film, and then with a whitereflective layer made of titania. This titania layer is stated to beporous enough for lithium ions to pass therethrough, but thelight-scattering properties of the titania produce a solid-whitereflector. The display is then filled with the inert electrolyte 110.

FIG. 2 of the accompanying drawings is a flow chart of a method(generally designated 200) for preparing the display 100 shown inFIG. 1. In a first step 202, a colloidal titania dispersion is preparedby hydrolysis of titanium tetraisopropoxide; titanium tetrachloride mayalternatively be used. The average diameter (7 nm) of the initiallyformed crystallites is increased to 12 nm by autoclaving at 200° C. for12 hours. The autoclaved dispersion is concentrated to a solids contentof 160 g/l and Carbowax 20000 (40% wt. equiv. of TiO₂) is added to yielda white viscous sol. (Carbowax 20000, a Registered Trade Mark, is anethylene glycol polymer of average molecular weight 20000.) Theresultant sol is then, in a step 204, printed on to a glass substratecarrying a conducting layer. More precisely, a 4 μm thick layer of thesol, 25 mm by 25 mm in size, is deposited using a screen-printingprinting technique on top of a 33 mm by 33 mm fluorine-doped tin oxidelayer on a glass substrate formed of Glastron (Registered Trade Mark).The resulting gel-film is dried in air for 1 hour, sintered at 450° C.for 12 hours and stored in a darkened vacuum desiccator prior to use.

Separately, in a step 206, a redox chromophore,N,N′-bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride, is prepared byadding 4,4′-bipyridyl (4.4 g) and diethyl 2-ethylbromophosphonate (15.0g) to water (75 ml). The reaction mixture is refluxed for 72 hours andallowed to cool. Following addition of concentrated hydrochloric acid(75 ml), the reaction mixture is refluxed for a further 24 hours. Torecover the desired product, the reaction mixture is concentrated to 50ml, isopropyl alcohol (200 ml) is added dropwise, and the mixture isstirred on ice for one hour and filtered. The white crystalline productis washed with cold isopropyl alcohol and air-dried to give pureN,N′-bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride.

In the next step 208 of the process, the redox chromophore prepared instep 206 is adsorbed on to the titania-coated substrate prepared in step204 above. More precisely, the titania films are modified by adsorptionof the redox chromophore from an aqueous solution (0.02 mol.dm-3) over24 hours, washed with distilled de-ionized water, dried in air, andstored in a darkened vacuum desiccator for 48 hours prior to use.

The remaining steps of the process may be most easily understood byreferring both to FIG. 2 and to FIGS. 3A and 3B. FIG. 3A shows, inschematic side elevation, an assembly (generally designated 300) beingused to form the complete display 100 (except for the electrolyte 110).The assembly 300 comprises a front electrode assembly (FIG. 3A isinverted for ease of comprehension) consisting of a first glasssubstrate 302 (which will eventually form the glass substrate 102), afirst tin-oxide coated conductive layer 304 (which will eventually formthe conductive layer 104), and a nano-structured film of titania 306(which will eventually form the film 106). The assembly 300 furthercomprises a rear electrode assembly, the construction of which will nowbe described with reference to FIGS. 3A and 3B.

FIG. 3B shows a top plan view of the pattern in which adhesive isapplied to the second substrate prior to the final assembly. The firststep 210 (FIG. 2) of the final assembly is the deposition, using ascreen printing technique, of a 2.5 mm border 116 of a commercial epoxyresin (Araldite—Registered Trade Mark) on to a second 33×33 mm glasssheet 308 provided with a layer 310 of fluorine-doped tin oxide; thissecond glass sheet 308 eventually forms the second glass substrate 114,while the tin oxide layer 310 eventually forms the second conductivelayer 112. As best seen in FIG. 3B, a small opening 118 is left in onecorner of the border 116. The adhesive-coated piece of conducting glassis placed on top of the first glass sheet bearing the modified titaniafilm prepared as described above and the resultant assembly is left for24 hours to enable the adhesive to set, thus forming the final assembly100, except for the absence of the electrolyte 110.

To complete construction of the electrochromic system, the abovesandwich structure is, in a step 212, back-filled under argon pressurewith an electrolyte solution consisting of lithium perchlorate (0.05mol. dm-3) and ferrocene (0.05 mol. dm-3) in γ-butyrolactone (m. pt.−45° C., b. pt. 204° C.). The components of the electrolyte solution arecarefully purified and rigorously dried prior to use. Finally, in a step214, the opening 118 is closed using the same commercial epoxy adhesiveas before.

In operation of the display 100, the nano-structured titania film 106colors on application of a potential sufficiently negative to accumulateelectrons in the available trap and conduction band states. According tothe aforementioned U.S. Pat. No. 6,301,038, as a consequence of the highsurface roughness of this nano-structured film, ions are readilyadsorbed/intercalated at the oxide surface, permitting efficient chargecompensation and rapid switching, i.e., the need for bulk intercalationis eliminated. However, despite the rapid switching times in such films,the associated change in transmittance is not sufficient for acommercial device. To overcome this limitation, redox chromophore 108 isadsorbed at the surface of the transparent nano-structured film; thischromophore, when reduced, increases the extinction coefficient of anaccumulated trapped or conduction band electron by more than an order ofmagnitude. Furthermore, due to the nano-porous structure and associatedsurface roughness of the nano-crystalline films used, the redoxchromophore is effectively stacked as in a polymer film, while at thesame time the intimate contact of the chromophore with the metal oxidesubstrate necessary to ensure rapid switching times is maintained.

One potentially important market for electro-optic “displays” (or ratherelectro-optic systems) is windows with variable light transmission. Asthe energy performance of buildings and vehicles becomes increasinglyimportant, electro-optic media could be used as coatings on windows toenable the proportion of incident radiation transmitted through thewindows to be electronically controlled by varying the optical state ofthe electro-optic media. Effective implementation of such“variable-transmissivity” (“VT”) technology in buildings is expected toprovide (1) reduction of unwanted heating effects during hot weather,thus reducing the amount of energy needed for cooling, the size of airconditioning plants, and peak electricity demand; (2) increased use ofnatural daylight, thus reducing energy used for lighting and peakelectricity demand; and (3) increased occupant comfort by increasingboth thermal and visual comfort. Even greater benefits would be expectedto accrue in an automobile, where the ratio of glazed surface toenclosed volume is significantly larger than in a typical building.Specifically, effective implementation of VT technology in automobilesis expected to provide not only the aforementioned benefits but also (1)increased motoring safety, (2) reduced glare, (3) enhanced mirrorperformance (by using an electro-optic coating on the mirror), and (4)increased ability to use heads-up displays. Other potential applicationsinclude of VT technology include privacy glass, angle-independenthigh-contrast large-area displays, glare-guards in electronic devices,and electronic scratchpads.

In order to increase the usability of electrochromic displays, there aresignificant challenges to overcome that will allow for improved productlife, lower costs, and wider manufacturing process conditions to producea larger portfolio of product types without significant increase inprocess design and hence increased costs and delay of entry into themarket.

A first aspect of the present invention seeks to reduce or eliminate onemajor limitation of the prior art electrochromic display described inthe aforementioned U.S. Pat. No. 6,301,038, WO 01/27690 and Woodarticle, namely that great care must be taken to contain the liquidelectrolyte and to create the seal. This step is expensive and can leadto product defects if the seal leaks, which is possible with changes intemperature (temperature cycling).

This first aspect of the present invention also seeks to reduce oreliminate another limitation of such prior art electrochromic displays,namely that the switching response of the display is limited to thespeed at which the ions can transport across the electrolyte, and thattherefore, such displays are limited to slow speed applications.

This first aspect of the present invention also seeks to reduce oreliminate another limitation of liquid electrolyte electrochromicdisplays, namely that it is difficult to create isolated cells unless anelectrolyte seal is used for each display cell element, which bothcauses great expense and limits density (i.e., how closely display cellscan be packed). When the cells are close together, ion transport of onecell can create ion transport of neighboring cells and therefore cancause the neighboring non-activated cells to become somewhat activated,resulting in a loss of clarity or resolution. In order to overcome thisin the current art, the cells are not packed closely together,preventing optimized resolution.

The first aspect of the present invention seeks to provide to create anelectrochromic display that is low cost, high density, fast, and longlasting, but which does not need a liquid electrolyte.

The first aspect of the present invention also seeks to provide astructure and method of making the structure that has a high-speed solidcharge transport layer between the display conductive elements and theredox promoter or chromophore. (Note that in such a display holes mustmove when electrons are used to produce the electrochromic effects. Ifoxidation is used to produce the electrochromic effects, the chargetransport layer can be an electron transport layer. Of course, holesmoving in one direction are equivalent to electrons moving in theother.)

The first aspect of the present invention also seeks to provide astructure and method of making the structure that has a high-speed solidcharge transport layer between the display conductive element and theredox promoter or chromophore where the redox promoter or chromophore isimproved for efficiency by chemically adding a charge transport polymerto the redox promoter or chromophore prior to adding the solid chargetransport layer.

The first aspect of the present invention also seeks to provide astructure and method of making the structure that has a high-speed solidcharge transport layer between the display elements and the redoxpromoter or chromophore where the solid charge transport layer ispatterned and aligned to the display elements.

A second aspect of the present invention is directed to increasing theusability of electrochromic displays in VT and other applications bymaking the displays more readable by providing better contrast betweenthe on/off states, as well as better contrast between different graylevels. Currently electrochromic displays are limited in contrast,because although the redox chromophores change color when ion transportoccurs, they do so against a clear electrolyte, and a thin (3-4 μm)roughened surface, which is not optimized for contrast, since it ischosen to enhance the amount of surface area for the connection of redoxchromophore and not for particular optical properties.

This second aspect of the present invention also seeks to increase therange of applications of electrochromic displays by providing a low-costelectrochromic display manufacturing process that integrates with otherprocesses such as flexible polymer substrate processes is required.Currently, electrochromic display manufacturing processes are timeconsuming (>12 hours of preparation time) and therefore are costly. Inaddition, because of the high-temperature (450° C.) titania sinteringstep, these processes do not integrate well with low-temperatureplastic-based processes for producing flexible displays.

The second aspect of the present invention seeks to provide a structurefor and method of making low-cost electrochromic displays. This secondaspect also seeks to provide a structure for and method of makingelectrochromic displays that has at least one of the followingadvantages: (a) less time consuming; (b) more easily integrated withother manufacturing processes; (c) providing displays that are morereadable and have enhanced contrast; (d) providing displays that have avery high surface roughness of nano-structured film to further enhancecontrast; and (e) providing displays that have both an improvedelectrolyte and a very high surface roughness of nano-structured film tofurther enhance contrast.

A third aspect of the present invention relates to reducing thesusceptibility of electrochromic displays to environmental factors.Application of electrochromic media in VT windows will necessarilyexpose the media to substantial variations in environmental conditions,and in the present state of electrochromic technology, the applicationsof electrochromic media are, the present inventors have realized,significantly limited by the susceptibility of such media to manyenvironmental factors, such as light, moisture, oxygen, andelectrostatic discharge. Reducing the susceptibility of the media tosuch factors would allow for improved product life, lower costs, andwider manufacturing process conditions to produce a larger portfolio ofproduct types without significant increase in process design and henceincreased costs and delay of market entry.

One major limitation of electrochromic media, the present inventors haverealized, is the susceptibility of the redox chromophore to lightdegradation. Under excessive ultra-violet (UV) radiation exposure (interms of total flux by either high doses or lower doses over longperiods), the redox chromophore may degrade or detach from thenano-structured titania film. Titania is notorious for itsphotocatalytic ability, under UV illumination, to catalyze the oxidationof organic materials, typically to carbon dioxide and water. Indeed,titania is used for precisely this reason in anti-microbial materials inair conditioning filters and medical devices, waste water treatment andair decontamination. Such photocatalytic oxidation of organic materialsis of course enhanced as the concentration of molecular oxygen in themedium increases. The photocatalytic oxidation is also enhanced as theconcentration of water in the medium increases, since titania canphotocatalytically split water to produce hydrogen and oxygen, so thatthe presence of water inherently produces an increase in oxygenconcentration. Furthermore, it is known that water may act as anucleophile under certain conditions and, in the presence of titania,water may nucleophilically attack the aromatic groups present in thechromophore, producing products of unknown chemistry which are likely tohave properties significantly different from those desired in thechromophore.

The susceptibility of components of the electrochromic medium todegradation by oxygen in the presence of titania is enhanced by theoxidation and reduction reactions which take place at the electrodesduring switching of the state of the medium, since it is well known thatmany compounds are more susceptible to such degradation reactions asthey are being generated at an electrode.

Another problem with electrochromic media, the present inventors haverealized, is their susceptibility to damage by discharges of staticelectricity, such as triboelectric charges built up during assembly of adisplay or its handling as it is moved to a desired location. Staticelectricity is typically of very high voltage, orders of magnitudelarger than the relatively small voltages, around 1 to 2 Volts, neededto switch electrochromic media. Static discharges can damage electronicparts and affect components of the media, especially where the dischargepasses through interfaces. Although the exact reactions involved are notcompletely understood, exposure of electrochromic media to large “overpotentials” beyond the working voltage of the media can degrade theredox chromophore by oxidation or reduction. Under such high voltagedischarges, the aromatic chromophores may be altered by, for example,intramolecular cyclizations and rearrangements and/or intermolecularcoupling or other similar reactions with adjacent chromophores.

Another possible problem with electrochromic media, the presentinventors have realized, is interaction between pixels. Theaforementioned U.S. Pat No. 6,301,038 describes direct driveelectrochromic displays having multiple pixels integrated together usinga common electrolyte seal. The seal used in the prior art to keep theelectrolyte inside the display is the border of epoxy resin. Becausegroups of pixels use a common electrolyte and because fields of onepixel are not well isolated from neighboring pixels, these non-isolatedfields could create ion transport near neighboring pixels. Hence, thesenon-isolated fields may modify the redox chromophore and further degradethe lifetime of the redox chromophore in that region (e.g., redoxchromophore designed to be “off' is “on” to a certain levelsystematically more often than it was designed to be).

Another problem with electrochromic media, the present inventors haverealized, is high fields caused by a lack of height or distance controlwithin an electrochromic display between the glass substrates, forexample as shown in FIG. 1. A user who presses on the first glasssubstrate moves the display surfaces closer together and, when inoperation, the field strength then increases significantly because thefield is proportional to the inverse of the distance. Hence, theelectrodes and other components of the display can degrade due toarcing, shorting, or excessive ion transport.

The third aspect of the present invention seeks to provide a means forand method of making a total environmental seal for electrochromicdisplays for sealing out unwanted light, unwanted moisture, unwantedoxygen (O2), unwanted electrostatic charge, and unwanted fields.

The first, second and third aspects of the present invention discussedabove all relate to improvements in electrochromic displays. However,the present invention has a fourth aspect which is applicable to allelectro-optic displays, not merely electrochromic displays, and thisfourth aspect of the present invention relates to a system and method ofoperation for integrating and controlling an electro-optic display.

As already mentioned electrochromic devices have been in use for sometime in relatively simple applications such as the electrochromic rearview mirrors for motor vehicles. These electrochromic mirrors changefrom the full reflectance mode (day) to the partial reflectance mode(s)(night) for glare-protection purposes from light emanating from theheadlights of vehicles approaching from the rear. However, recentresearch into the production of nano-crystalline electrochromic displayelements based on chemically modified nano-structured meso-porous filmsindicates that electrochromism can be extended to the high densityelectronic display market and can favorably compete with LCD's incertain applications.

Also as already mentioned, U.S. Pat. No. 6,301,038 describes anelectrochromic display in which the active layer (i.e., the layer whoseoptical characteristics are varied by addition or removal of electronsis a nano-porous-nano-crystalline film comprising a semi-metallic oxidehaving a redox chromophore adsorbed thereto. Also as already mentioned,this patent describes a nano-crystalline electrochromic systemcomprising a first electrode disposed on a transparent or translucentsubstrate and a second electrode, an electrolyte, an electron donor anda nano-porous-nano-crystalline film of a semiconducting metallic oxidehaving a redox chromophore adsorbed thereto being intermediate the firstand second electrodes. In FIG. 4 is illustrated a nano-crystallineelectrochromic display cell 400 similar to those described in thispatent. Nano-crystalline electrochromic display cell 400 consists of afirst glass substrate 410, a fluorine-doped tin-oxide coated conductivelayer 420, a terminal 425, a nano-structured film of titania 430, aredox chromophore 440, an electrolyte solution 450, a conductive element460, a terminal 465, a conductive element 470, a terminal 475, and asecond glass substrate 480.

Redox chromophore 440 is colorless in the oxidized state and colored inthe reduced state. It is linked to the surface of nano-structuredtitania 430, a nearly colorless semiconductor, on the first glasssubstrate 410. When a current is allowed to flow from terminal 465 toterminal 425, electrons are injected from first glass substrate 410 intothe conduction band of the semiconductor nano-structured titania film430, and this current reduces the redox chromophore 440. This reductionis reversible: application of a reverse current re-bleaches the redoxchromophore 440 by oxidation.

A major potential advantage of nano-crystalline electrochromic displaytechnology is the ability to create a true “paper-like” display. Acombination of high reflectivity and achievable contrast ratio gives thenano-crystalline electrochromic display an appearance that is more likeink on paper than most other display technologies available and that canbe read at very large angles to the perpendicular (again, like paper).An additional advantage is that nano-crystalline electrochromic displayscan be made bi-stable, meaning that once switched on, a pixel will staycolored until actively bleached. In other words, no power is consumed tokeep a pixel colored. This, combined with the fact that the display isreflective, needing no backlighting, means that the displays can bedesigned to require very little power to operate. This could provide asignificant market advantage in handheld, battery operated electronicdevices such as cellular phones, PDA's, and electronic books.

Nano-crystalline electrochromic-based display cells have been studied asindividual cells, under simple direct drive operation, whereas none ofthe known art discusses active matrix displays. Indeed, it is not quiteclear how to make active matrix nano-crystalline electrochromic displayssince a common electrolyte that is used between cells may causeinterference between cells in an active matrix.

Even in direct-drive nano-crystalline electrochromic displays, thesystem (electronics/software/display) and method of operation arecomplex because, as in the electrophoretic display cell discussedearlier, the state of the cell before it is written is critical indetermining how to change the cell state.

FIG. 5 shows a simplified electrical circuit model (generally designated500) of the nano-crystalline electrochromic display cell 400 shown inFIG. 4. Electrical circuit model 500 consists of terminal 465 andterminal 425 as described above with reference to FIG. 4, a resistor510, a resistor 520, and a capacitor 530.

Resistor 510 represents the summed series impedance of terminal 465,terminal 425, conductive layer 420, nano-structured titania film 430,redox chromophore 440, conductive element 460, and terminal 465.Capacitor 530 represents the capacitance of electrolyte solution 450.Resistor 520 represents the sum of the impedance as represented byresistor 510 and the series impedance of electrolyte solution 450.Resistor 520 has a characteristic impedance that is much larger thanthat of resistor 510. It should be noted that a considerably morecomplex small-signal model could be developed for nano-crystallineelectrochromic display cell 400. Of special interest would be inclusionof the variable diodic behavior of nano-structured titania film 430.

In order to take competitive advantage of the potential of electro-opticdisplay technologies, and in particular of the encapsulatedelectrophoretic, rotating bichromal member and electrochromic displaytechnologies described above, the displays need to be more readable, andhence require better contrast between the “on/off” states as well asbetter contrast between different “gray levels”. A key technique inachieving this is to drive the electro-optic display in a manner whichtakes into account environmental factors. For example, thenano-crystalline electrochromic display cell 400 should be driven in amanner that takes into account many factors. The nano-crystallineelectrochromic display cell 400 drive system must account for (1) thesteady-state response of resistor 510, resistor 520, and capacitor 530;(2) the time-varying response of resistor 510, resistor 520, andcapacitor 530; (3) interactions with adjacent cells; (4) lightreflectivity vs. ion transport curve for redox chromophore 440; (5) iontransport efficiency; (6) variable diodic behavior of nano-structuredtitania film 430; (7) interaction of optical feedback; (8) electrolytepotential changeover; (9) changes over operating life of electrolytesolution 450 performance; (10) changes over operating life of redoxchromophore 440 performance; (11) effects of ambient and operatingtemperatures on cell performance; and (12) effects of ambient light ondisplay quality.

Similarly, it has been observed that the optical characteristics ofencapsulated electrophoretic media vary as a function of temperature andhumidity, and the “age” of the medium; this aging phenomenon is affectedby both the chronological age of the medium, that it to say the periodsince the medium was manufactured, and the “operating age”, that is tosay the period for which the medium has been driven. More specifically,the electrical resistivity of an encapsulated electrophoretic mediumvaries inversely with temperature, decreasing as the temperatureincreases. This variation of electrical resistivity with temperatureaffects how much current passes through the medium when it is drivenwith a constant drive pulse, and this is turn affects the rate at whichthe medium switches and the rate at which the medium ages during use.Thus, using a fixed drive pulse with an encapsulated electrophoreticmedium which is undergoing changes in ambient temperature and humidity,and is also aging, can lead to substantial and undesirable changes inthe optical properties of the medium. Such changes can include thereflectances of the white and dark states of the medium and theintermediate gray states (if any), and hence also the contrast ratio ofthe medium. For example, the medium may show acceptable properties whenswitched at room temperature, but its contrast ratio may be reduced whenoperating at low temperatures, and the medium may be over-saturatedand/or over-driven at high temperatures; such over-driving isundesirable because it tends to reduce the working lifetime of themedium.

Changes in environmental conditions may also cause problems withself-erasing of the medium. “Self-erasing” (see, for example, Ota, I.,et al., “Developments in Electrophoretic Displays”, Proceedings of theSID, 18, 243 (1977), where self-erasing was reported in anunencapsulated electrophoretic display) is a phenomenon whereby, whenthe voltage applied across certain electrophoretic media is switchedoff, the electrophoretic medium may reverse its optical state, and insome cases a reverse voltage, which may be larger than the operatingvoltage, can be observed to occur across the electrodes adjacent themedium. It appears (although this invention is in no way limited by thisbelief) that the self-erasing phenomenon is due to a mismatch inelectrical properties between various components of the display.Obviously, self-erasing is highly undesirable in that it reverses (orotherwise distorts, in the case of a grayscale medium) the desiredoptical state of the medium.

Similar problems are encountered with other types of electro-opticmedia. For example, the switching characteristics of rotating bichromalmember media will vary with temperature due to changes with temperaturein the viscosity of the liquid medium which surrounds the rotatingbichromal members, and such temperature-dependent changes may affect thegray scale of the medium.

In its fourth aspect, the present invention seeks to provide a systemand method of operation for integrating and controlling an electro-opticdisplay.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display to provide controlled dark and whitestates, and controlled intermediate states (gray scale) in displayscapable of such gray scale.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display for optimum steady-state performance.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display for optimum time-varying performance.

The fourth aspect of the present invention also seeks to electricallydrive a nano-crystalline electrochromic display cell that compensatesfor interference from adjacent cells.

The fourth aspect of the present invention also seeks to electricallydrive a nano-crystalline electrochromic display cell that compensatesfor variations in light reflectivity vs. ion transport curve for redoxchromophores.

The fourth aspect of the present invention also seeks to electricallydrive a nano-crystalline electrochromic display cell that compensatesfor variations in ion transport efficiency.

The fourth aspect of the present invention also seeks to electricallydrive a nano-crystalline electrochromic display cell that compensatesfor variations in diodic behavior of a nano-structured titania film.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display that compensates for the interaction ofoptical feedback.

The fourth aspect of the present invention also seeks to electricallydrive a nano-crystalline electrochromic display cell that compensatesfor electrolyte potential changeover.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display in a manner that incorporates thematerial aging aspects of the electro-optic medium.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display in a manner that compensates for theeffects of ambient and operating temperatures on electro-optic mediumperformance.

The fourth aspect of the present invention also seeks to electricallydrive an electro-optic display in a manner that compensates for ambientlight conditions.

SUMMARY OF INVENTION

In its first aspect, the present invention provides an electrochromicdisplay, of the type described in the aforementioned U.S. Pat. No.6,301,038, WO 01/27690 or Wood article, and comprising anelectrochromically-active layer comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto. The display of the presentinvention has a solid charge transport layer disposed adjacent the filmand in charge transport relationship therewith. In one form of thisdisplay, the charge transport layer is interrupted between adjacentpixels of the display.

The first aspect of the present invention also provides anelectrochromic display, of the type described in the aforementioned U.S.Pat. No. 6,301,038, WO 01/27690 or Wood article, and comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto, the display having a chargetransport material bonded to the promoter or chromophore.

The second aspect of the present invention provides an electrochromicdisplay, of the type described in the aforementioned U.S. Pat. No.6,301,038, WO 01/27690 or Wood article, and comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto, the display having alight-scattering material dispersed in the electrolyte. Thislight-scattering material is conveniently titania.

The second aspect of the present invention also provides anelectrochromic display of this type having a light-scattering materialdisposed over the metal oxide layer and on the opposed side of thislayer from the substrate carrying the metal oxide layer.

The second aspect of the present invention also provides anelectrochromic display of this type having optical gaps of about 0.2 toabout 0.4 μm in diameter in the electrochromically-active layer.

The second aspect of the present invention also provides a method forpreparing such an electrochromic display having such gaps, this methodcomprising admixing particles of the conducting metal oxide withsacrificial particles, coating the mixture of the metal oxide particlesand the sacrificial particles on to a substrate to form a layer of themixed particles and thereafter removing or destroying the sacrificialparticles, leaving a layer of the metal oxide particles with the opticalgaps therein.

The second aspect of the present invention also provides anelectrochromic display of this type having an electrochromically-activelayer comprising metal oxide particles coated with a material,preferably silica and/or alumina, capable of being sintered at atemperature below about 400° C.

The second aspect of the present invention also provides a method forpreparing such an electrochromic display, this method comprising coatingthe metal oxide particles with a material capable of being sintered at atemperature below about 400° C., coating a layer of the coated metaloxide particles on a substrate, and thereafter sintering the layer ofcoated metal oxide particles at a temperature below about 400° C. In apreferred embodiment of this method, the sintering is conducted at atemperature below about 150° C. In another preferred embodiment of thisprocess, the sintering is effected by liquid phase sintering withpressure, preferably using a roll-to-roll drum process; some variants ofthis preferred embodiment can be carried out at ambient temperature.

The third aspect of the present invention provides an electrochromicdisplay, of the type described in the aforementioned U.S. Pat. No.6,301,038, WO 01/27690 or Wood article, and comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto, the display having light sealingmeans arranged to reduce or eliminate exposure of theelectrochromically-active layer to radiation, thereby reducing oreliminating the light degradation of the redox chromophore due tophotocatalytic reactions at the electrochromically-active layer.

The third aspect of the present invention also provides anelectrochromic display of this type having oxygen sealing and/orremoving (“getting”) means for providing an oxygen seal and/or forremoving oxygen from within the display in order to stop or minimize thedegradation of the electrolyte due to exposure of the electrolyte tooxygen.

The third aspect of the present invention also provides anelectrochromic display of this type having water sealing and/or removing(“getting”) means for providing a water seal and/or for removing waterfrom within the display in order to stop or minimize the degradation ofthe electrolyte due to exposure of the electrolyte to water.

The third aspect of the present invention also provides anelectrochromic display of this type having electrostatic dischargeprevention means for protecting against electrostatic discharge thatwould degrade the electrolyte and/or damage theelectrochromically-active layer of the display.

The third aspect of the present invention also provides anelectrochromic display of this type having means to isolate eachelectrochromic pixel from unwanted fields.

The present invention also provides an electrochromic display of thistype having means to control the distance between the electrode surfacesof the display to minimize or eliminate high fields, thus removing orreducing the tendency of such high fields to degrade the electrolyteand/or electrochromically-active layer of the display.

The fourth aspect of the present invention provides an electro-opticdisplay comprising:

an electro-optic medium;

at least one electrode arranged to apply an electric field to theelectro-optic medium;

drive means for supplying a driving pulse to the electrode;

a temperature sensor for sensing the temperature of, or adjacent to, theelectro-optic medium and producing an output signal representative ofthe temperature; and

control means for receiving the output signal from the temperaturesensor and controlling the drive means to vary the driving pulsedependent upon the output signal.

The fourth aspect of the present invention also provides anelectro-optic display comprising:

an electro-optic medium;

at least one electrode arranged to apply an electric field to theelectro-optic medium;

drive means for supplying a driving pulse to the electrode;

a humidity sensor for sensing the humidity of, or adjacent to, theelectro-optic medium and producing an output signal representative ofthe humidity; and

control means for receiving the output signal from the humidity sensorand controlling the drive means to vary the driving pulse dependent uponthe output signal.

The fourth aspect of the present invention also provides anelectro-optic display comprising:

an electro-optic medium;

at least one electrode arranged to apply an electric field to theelectro-optic medium;

drive means for supplying a driving pulse to the electrode;

a timer for measuring the operating time of the electro-optic medium andproducing an output signal representative of this operating time; and

control means for receiving the output signal from the timer andcontrolling the drive means to vary the driving pulse dependent upon theoutput signal.

The fourth aspect of the present invention also provides a method ofoperating an electro-optic display, the display comprising anelectro-optic medium and at least one electrode arranged to apply anelectric field to the electro-optic medium, the method comprising:

applying a first driving pulse to the electrode;

measuring the optic state of at least one portion of the electro-opticmedium after application of the first driving pulse thereto; and

applying a second driving pulse to the electrode, the second drivingpulse being controlled by the measured optical state of the at least oneportion of the electro-optic medium.

The fourth aspect of the present invention also provides a method ofoperating an electro-optic display, the display comprising anelectro-optic medium and at least one electrode arranged to apply anelectric field to the electro-optic medium, the method comprising:

applying a first driving pulse to the electrode;

measuring the current passing through the electro-optic medium as aresult of the application of the first driving pulse; and

applying a second driving pulse to the electrode, the second drivingpulse being controlled by the measured current resulting from the firstdriving pulse.

BRIEF DESCRIPTION OF DRAWINGS

As already mentioned:

FIG. 1 of the accompanying drawings is a schematic cross-section througha first display of a type described in the aforementioned U.S. Pat. No.6,301,038;

FIG. 2 is a flow chart of a method for preparing the display shown inFIG. 1;

FIG. 3A is a schematic side elevation of an assembly used to form thedisplay shown in FIG. 1;

FIG. 3B is a top plan view of part of the assembly shown in FIG. 3A;

FIG. 4 is a schematic cross-section through one cell of a second displayof the type described in the aforementioned U.S. Pat. No. 6,301,038; and

FIG. 5 shows a simplified electrical circuit model of the cell shown inFIG. 4. The remaining Figures of the accompanying drawings are asfollows:

FIG. 6 is a schematic cross-section, generally similar to that of FIG.4, through one cell of an electrochromic display in accordance with thefirst aspect of the present invention and having a solid chargetransport layer;

FIG. 7 a flow chart, similar to that of FIG. 2, of a method forpreparing the display shown in FIG. 6;

FIG. 8 is a schematic cross-section, generally similar to those of FIGS.4 and 6, through part of an electrochromic display in accordance withthe first aspect of the present invention and having a solid chargetransport layer which is interrupted between adjacent pixels;

FIG. 9 is a schematic cross-section, generally similar to those of FIGS.4, 6 and 8 through one cell of a third display of the type described inthe aforementioned U.S. Pat. No. 6,301,038;

FIG. 10 is a schematic cross-section, generally similar to that of FIG.9, through one cell of a display similar to that shown in FIG. 9 butmodified in accordance with the second aspect of the present invention;

FIGS. 11 and 12 are schematic side elevations of nano-porousnano-crystalline titania layers which may be used in displays inaccordance with the second aspect of the present invention;

FIG. 13 is a schematic cross-section, generally similar to that of FIG.10, through one cell of a second display in accordance with the secondaspect of the present invention;

FIG. 14A is a three-quarter perspective view of an apparatus for formingnano-porous nano-crystalline titania layers without sintering naccordance with the second aspect of the present invention;

FIG. 14B is a schematic side elevation of the portion of the apparatuswithin circle B in FIG. 14A;

FIG. 14C is a schematic side elevation of the portion of the apparatuswithin circle C in FIG. 14A;

FIG. 15 is a schematic cross-section, generally similar to that of FIG.13, through one cell of a display protected against ultra-violetradiation in accordance with the third aspect of the present invention;

FIG. 16 is a schematic cross-section, generally similar to that of FIG.13, through one cell of a display protected against moisture inaccordance with the third aspect of the present invention;

FIG. 17 is a top plan view, similar to that of FIG. 3B, showing themanner in which the seals of the display shown in FIG. 16 are formed;

FIG. 18 is a schematic cross-section, generally similar to that of FIG.13, through one cell of a display protected against high internal fieldsin accordance with the third aspect of the present invention;

FIG. 19 is a block diagram of an electrochromic display and itsassociated driving circuitry in accordance with the fourth aspect of thepresent invention;

FIG. 20 is a flow chart of a method for driving an electrochromicdisplay in accordance with the fourth aspect of the present invention;

FIG. 21 is a flow chart of a method for assessing the state of anelectrochromic display in accordance with the fourth aspect of thepresent invention;

FIG. 22 is a flow chart of a method for applying rules for driving anelectrochromic display in accordance with the fourth aspect of thepresent invention; and

FIGS. 23 and 24 illustrate circuits used in a temperature-compensatedvoltage supply for driving an encapsulated electrophoretic display inaccordance with the fourth aspect of the present invention.

DETAILED DESCRIPTION

As already mentioned, the present invention has four main aspects, whichwill be described separately below. However, it will appreciated thatthe various aspects of this invention may be used singly or incombination. For example, an electrochromic display which makes use ofthe second aspect of the present invention for improving the contrast ofthe display may also make use of the third aspect of the presentinvention for sealing the display from the outside environment, and mayalso make use of the fourth aspect of the present invention by using adrive scheme which is adjusted to take account of certain environmentalparameters.

Section A: Display with Solid Charge Transport Layer

In order to produce a low-cost, high-contrast, faster switchingelectrochromic display with a process that allows for a flexible displaytechnology, in accordance with first aspect of the present invention,the liquid-based electrolyte used in prior art displays is replaced witha solid charge transport layer. By removing the liquid electrolyte, lessconcern over sealing the display. and ensuring that the electrolyte isuniformly covering the display, is needed. Because of the absence of aliquid electrolyte, manufacturing is simpler and thus cheaper. Inaddition, because of the presence of the solid charge transportmaterial, the display's switching speed is increased, since theswitching speed is no longer limited by the slow movement of ions in theliquid electrolyte.

This aspect of the present invention has three main embodiments. In thefirst embodiment, liquid electrolyte is replaced by a charge transportlayer. In the second embodiment, a charge transport material is bondedto the redox-based promoter or chromophore (e.g. viologen) before thepromoter or chromophore is incorporated into the display. The thirdembodiment may make use of the features of either of the first twoembodiments, but the solid charge transport layer is aligned to thedisplay element (pixels) so that each pixel is isolated from adjacentpixels. In all cases, an electrolytic seal to keep a liquid electrolyteconfined within the display is not required, thus saving cost andallowing for integration with flexible substrates.

Section A1: First Embodiment—Solid Charge Transport Layer

FIG. 6 shows a schematic cross-section through a display (generallydesignated 600) having a front section 602 (through which an observertypically views the display 600) and a rear section 604. As in the priorart described above, the front section 602 includes a first glasssubstrate 606, an ITO conductive layer 608, a titania nano-crystallinelayer 610, and a redox chromophore (or promoter) layer (e.g., viologen)612. The rear section 604 includes a second glass substrate 614, andtransparent conductive ITO display elements 616 and 618. Not shown areelectrical terminals that attach to the conductive layer 608 and toconductive display elements 616 and 618.

The display 600 further includes, in accordance with the first aspect ofthe present invention, a solid charge transport layer 620, comprising,for example triphenylamine, which acts in place of the prior artelectrolyte between front section 602 and rear section 604, and theirrespective electrodes, the ITO conductive layer 608 and the displayelements 616 and 618. The charge transport layer 620 may be solvent castby forming a solution of the charge transport material in a solution ofa polymer, and the resultant solution coated in a layer, which istypically 5 to 200 μm thick, thinner layers being generally preferred,and the layer finally dried.

FIG. 7 shows a flow chart of a method 700 for forming the electrochromicsystem shown in FIG. 6. Steps 202-208 in FIG. 7 are identical to thosein FIG. 2; the remaining steps are as follows:

Step 760: Preparing Solution of Charge Transport Material

In this step, a charge transport material (e.g., triphenylamine) isdissolved in a suitable organic solvent, such as acetonitrile, tolueneor chloroform.

Step 770: Forming Solid Charge Transport Layer

In this step, the solution prepared in Step 760 is coated on the frontsection 602 by any convenient technique, for example spin coating, dipcoating or stencil coating, and dried to produce the final solid layer.The film may alternatively be cast with a dissolved polymer (forexample, polystyrene) in the toluene solution to provide bettermechanical integrity in the final solid layer.

Step 780: Assembly

In this step, a glue layer bead is added to the edges of either or bothof the front section 602 and the rear section 614, and the two sectionsare assembled by being pressed together. Silicone adhesives mayconveniently be used.

It will be understood by those skilled in the art that other chargematerials may be used, for example triphenyldiamine, phthalocyanines,metal coordinated phthalocyanines, 4,4′,4″-tris(N,N-diphenylamine). Awide variety of charge transport materials are known for use inxerographic copying and the suitability of any of these materials foruse in the present invention may readily be determined by routineempirical tests.

Section A2: Second Embodiment: Charge Transport Material Bonded toPromoter or Chromophore

In the second embodiment of the first aspect of the present invention, acharge transport material is bonded to the redox promoter orchromophore.

Although other methods of bonding the charge transport material to theredox promoter or chromophore may be used if desired, in a preferredform of the second embodiment, the redox promoter or chromophore isfirst modified to contain an amino group, if such a group is not alreadypresent. The amino-containing promoter or chromophore is then reactedwith a charge transport material containing (or modified to contain) analdehyde, acyl halide or carboxylic acid group to produce a product inwhich the promoter or chromophore is linked via an amide linkage to thecharge transport material.

Alternatively, the promoter or chromophore may be linked to the chargetransport material via an ether linkage, for example by a Williamsonether coupling reaction, by the reaction of a hydroxide or thiol, in thepresence of base, with an alkyl or aryl halide functional group. Otherpossibilities include the formation of an amine linkage by the reactionof a primary amino group with an alkyl or aryl halide group. Arylamines, phenols and any thiols may be reacted with aryl halides in thepresence of a metal catalyst, such as a palladium-phosphine basedcatalyst, to assemble aryl ether, aryl thiol and aryl amine linkages(the Buchwald-Hartwig chemistry). As will be apparent to those skilledin the art, in all cases which reactive group is provided on the chargetransport material and which is present on the promoter or chromophoreis essentially a matter of synthetic convenience, availability ofstarting materials, presence of interfering groups on a particularreactant and similar factors which will be familiar to organic syntheticchemists. It will also be understood that there are numerous other waysto bond the charge transport material to the promoter or chromophore.

The charge transport material may be bonded to the promoter orchromophore before this promoter or chromophore is incorporated into thedisplay, or the bonding of the charge transport material to the promoteror chromophore may be effected after the promoter or chromophore hasbeen adsorbed on to the titania. In the latter case, the titania layercould be immersed in an organic solution containing the modified chargetransport material and any other reagents (for example, a base) neededfor the necessary reaction to occur. Heat might be applied to acceleratethe reaction.

The foregoing description assumes that promoter or chromophore will liebetween the charge transport material and the titania, i.e., that thestructure will be titania-promoter/chromophore-charge transportmaterial. However, at least in some cases, it may be advantageous toinsert the charge transport material between the titania and thepromoter or chromophore, thus producing the structure titania-chargetransport material-promoter/chromophore. To prepare such a structure,the charge transport material and the promoter or chromophore may belinked in any of the ways previously described, but it is necessary toprovide the charge transport material with a group which will absorb onthe titania, and desirable to modify the promoter or chromophore so thatit does not strongly absorb on the titania.

Section A3: Third Embodiment—Defined Regions in the Solid ChargeTransport Layer

As already mentioned, the third embodiment of the first aspect of thepresent invention relates to the use of defined (separate) regions in asolid charge transport layer, and in particular the use of a solidcharge transport layer which is interrupted between adjacent pixels ofthe display. FIG. 8 shows a display (generally designated 800), which isgenerally similar to the display 600 shown in FIG. 6; in particular, thedisplay 800 comprises glass substrates 606 and 614, an ITO conductivelayer 608, a titania nano-crystalline layer 610, a redox chromophorelayer 612 and display elements 616 and 618, all of which are essentiallythe same as the corresponding elements in the display 600 shown in FIG.6. The display 800 further comprises a solid charge transport layer 820,which is generally similar to the corresponding charge transport layer620 shown in FIG. 6, but which differs therefrom by being interrupted bya gap 810 between display elements 616 and 618; although this is notapparent from FIG. 8, multiple gaps 810 are provided within the chargetransport layer 820 so that each pixel of the display (each pixel beingdefined by a display element such as the elements 616 and 618) isisolated from all adjacent pixels. The gap 810 may be filled with eitherair or a clear material, typically a polymer, which is free from chargetransporting materials (i.e., is an insulator).

The display 800 may be manufactured by a method substantially the sameas the method 700 shown in FIG. 7 but with the following additionalsteps inserted after step 770 and before step 780:

Step 771: Applying Imaging Layer on Solid Charge Transport Layer

In this step, a photo resist or photo-imageable polymer is spin appliedon top of the solid layer of charge transport material 820.

Step 772: Defining Image in Imaging Layer

In this step, images are exposed and developed in the photo resist orphoto-imageable polymer imaging layer.

Step 773: Defining Aligned Image in Imaging Layer

In this step, images that align with display elements 616 and 618 areexposed and developed in the photo resist or photo-imageable polymerimaging layer.

Step 774: Etching Charge Transport Layer

In this step, charge transport layer 820 is etched using a wet or dryetch technique, depending upon the specific charge transport materialused, patterning the layer 820 and leaving gap 810.

Step 775: Removing Imaging Layer

In this step, the photo resist or photo-imageable polymer imaging layeris removed using an organic solvent which dissolves the photo resist orphoto-imageable polymer but not the charge transport material.Alternatively, the photo resist or photo-imageable polymer could beremoved by dry etching to expose the charge transport layer 820.

Step 776: Fill Gaps

In this optional step, gap 810 is filled with any suitable insulator.

It will be appreciated that the first aspect (and indeed the otheraspects) of the present invention may make use of any of the materials,processes and techniques described in the aforementioned U.S. Pat. No.6,301,038, WO 01/27690 and Wood article, provided of course that thematerials are compatible with the use of solid charge transport layers.In this connection, it is noted that many of the preferred electroactivecompounds described in WO 01/27690 for use in nano-porous,nano-crystalline films are already provided with carboxylic acid groups,which can be used to link these electroactive compounds to chargetransport materials by the chemistry already described.

Section B: Display with Improved Contrast and Readability

As already mentioned, the second aspect of the present invention relatesto improvements over the prior art in terms of enhanced contrast and,hence, readability for the user. This aspect of the invention providesstructures for and methods of making high-contrast, low-costelectrochromic displays.

In one form of the second aspect of the present invention, as alreadymentioned, a light-scattering material, such as titania, is added to theelectrolyte of the display. The light-scattering material enhances thecontrast of the display by making off regions of the display a whiterand/or more consistently whiter color.

In another form of the second aspect of the present invention, asalready mentioned, physical gaps are created in theelectrochromically-active layer by using a sacrificial spacer particlesuch as polystyrene, latex or silica particles to alter themicrostructure (i.e., the structure on the micro scale) of thenano-structured film. The physical gaps enhance the contrast of the offregions of the display by scattering light of the wavelength that makesthe background whiter. The gaps leads to better absorption andre-emission of light, which further enhances the display contrast.

In another form of the second aspect of the present invention, asalready mentioned, a layer of a light-scattering material, preferablytitania, is provided on the opposed surface of theelectrochromically-active layer from the substrate carrying this activelayer, so that the light-scattering material lies behind the activelayer as seen by an observer. This layer of light-scattering materialfurther enhances the display contrast.

In addition, the second aspect of the present invention provides animprovement of the art in terms of lowering the cost and temperature ofthe manufacturing process, so that more applications are possible, e.g.flexible displays that use plastic-based substrates. This aspect of theinvention provides a structure for and method of making a low-cost andlow-temperature electrochromic display device. In one embodiment, thenano-crystalline film is created with titania or similar particlescoated with a layer of a material which can be sintered at lowtemperatures to provide a simplified lowered temperature process.

These various improvements provided by the second aspect of presentinvention can be used singly or in any combination to enhance thecontrast of the display.

Section B1: First Embodiment—Improving Electrolyte Contrast

The preferred electrolyte used in the prior art electrochromic displaysdiscussed above is optimized for its ion transport properties, but isnot optimized for contrast against the activated redox chromophore (achromophore that has accepted an electron through a redox reaction andhas undergone a color change). As more ion transport occurs through theelectrolyte in the regions where electrons are available, more redoxchromophore is activated. Thus, the electrochromic display contrast islimited to electron availability (ion transport) only and the redoxchromophore, against a non-optimized electrolyte background (i.e., inregions where the electrolyte is not activated), is not optimized forits optical properties for background light reflection.

FIG. 9 is a simplified view of a prior art electrochromic display(generally designated 900) comprising a first glass substrate 906, afirst fluorine-doped, tin-oxide-coated conductive layer 908, anano-structured titania film 910, a redox chromophore 912, anelectrolyte solution 920 (also known as electron donor solution), one ormore conductive elements 916, and a second glass substrate 914.

It is not clear from the descriptions in the aforementioned U.S. Pat.No. 6,301,039 and WO 01/27690 which is intended to the viewing surfaceof the display, nor is it clear whether the display is intended to beviewed in transmission or reflection. However, according to theaforementioned Wood article, in the presently-preferred display thesubstrate carrying the electrochromically-active layer provides theviewing surface (i. e., the display is viewed from below in FIG. 9), andthis layer is viewed in against a background provided by the whitereflective titania coating on the opposed “rear” electrode, so that thedisplay operates in a manner rather similar to a reflective LCD, in thatthe optically-active layer is transmissive, but a reflector provided onthe opposed side of the optically-active layer from the observer, sothat the observer views the display in a reflective mode. Where thereflective layer is placed will affect the contrast of the display. Openspaces in the micro-structured film will improve its optical propertiesby keeping light in the layer for multiple scattering events; theimprovement achieved is similar to the difference between the colorachieved by a color filter and a layer of paint.

When viewing electrochromic display 900, regardless of whether or not areflective layer mentioned in the Wood article is provided on the secondglass substrate 914, poor contrast may exist between region A and regionB, because the viewer must look through electrolyte solution 920.

In order to improve the contrast of the display, titania particles maybe dispersed within the electrolyte solution 920, and a display modifiedin this manner is illustrated in FIG. 10.

FIG. 10 shows an electrochromic display (generally designated 1000)comprising a first glass substrate 1006, a conductive layer 1008, anano-structured titania film 1010, a redox chromophore 1012, anelectrolyte solution 1020, conductive elements 1016, and a second glasssubstrate 1014, all of which are essentially identical to thecorresponding integers described above with reference to FIG. 9. Aplurality of titania particles 1022 are dispersed throughout theelectrolyte solution 1020.

Titania particles 1022 are 100-500 nanometer particles added to theelectrolyte solution 1020 in an amount enough such that these particles,if dried over a surface, would create a 0.5 μm to 1 μm thick film.Titania particles 1022 in this amount are added to electrolyte solution1020 before electrolyte solution 1020 is filled into electrochromicdisplay 1000. Titania particles 1022 enhance the contrast between regionA and activated region B because an electrolyte solution 1020 withtitania particles 1022 reflects more light and appear whiter than theelectrolyte solution 920 (FIG. 9) without titania particles.

In order to improve the dispersion of titania particles 1022 withinelectrolyte solution 1020, a dispersive agent may be added to themixture of electrolyte solution 1020 and titania particles 1022. Thedispersive agent can be an ionic surfactant such as sodiumdodecylsulphate (SDS) or non-ionic surfactant such as Triton-X 100. Thedispersive agent could also be an organic solvent such as acetonitrile.

Another way to improve the dispersion of titania particles 1022 inelectrolyte solution 1020 is to coat titania particles 1022 with a thin,5-10 nm coating of a non-adhering polymer, where the non-adheringpolymer coating is slightly charged to ensure that the particles(non-adhering polymer-coated titania) do not stick together. Suchpolymer coating could be accomplished by dispersing the titania in aaqueous solution of a poly-ionomer or polyelectrolyte such aspoly(styrenesulfonic acid sodium salt), or a copolymer. An example of anappropriate copolymer would be a co-polymer formed from acrylic acid(sodium salt) and acrylic or poly(acrylic acid (sodiumsalt)-co-acrylamide). The titania particles could also be polymer-coatedand/or charged by any of the methods described in the aforementioned2002/0185378.

Another way to enhance the contrast of electrolyte solution 1020 is touse both polymer-coated titania particles 1022 and an added dispersiveagent.

White or light-colored particles other than titania may of course beused. For example, barium sulfate particles, kaolin (clay particles), orlead oxide particles could be used instead of titania particles 1022.

Still another way to improve the contrast of electrolyte solution 1022is to add to electrolyte solution 1020 particles of a color thatenhances the contrast relative to the color of the activatedchromophore. For instance, if the chromophore transmits blue, it isbetter to add white particles than blue particles to electrolytesolution 1020.

Section B2: Second Embodiment—Enhancing the Contrast of the SurfaceRoughness by Creating Gaps

In the second embodiment of the second aspect of the present invention,in order to improve the contrast between the activated regions andnon-activated regions of an electrochromic display, the roughness ofnano-structured titania film 910 (FIG. 9) is enhanced to absorb andre-emit the wavelength of visible light. The resultant enhancedabsorption and re-emission leads to higher contrast regions compared tothe activated regions B (FIG. 9).

Typically, in the aforementioned U.S. Pat. No. 6,301,038, WO 01/27690and Wood article, the nano-structured titania film 910 is 3-4 μm thickwith a surface area to unit planar area of 600-1000.

Microscopically, as shown in FIG. 11, a nano-structured titania film(generally designated 1100) has a very rough, surface and themicrostructure comprises connected titania flakes. In order to improvethe contrast of film 1100, in accordance with the present inventionrandomized optical gaps, which are 0.2-0.4, preferably 0.2-0.3 μm, areadded in the titania. These optical gaps serve to absorb and re-emitlight of the correct wavelength, thereby improving the contrast. Opticalgaps in a solid scatter light in the same way as do bubbles in a liquid.Gaps of the same order of magnitude as the wavelengths of visible lightact as resonators for the alternating electric and magnetic fields ofvisible electromagnetic radiation. The resonance of light in these gapsemits light in all directions, and this re-emitted light passes backthrough the layer generating a deeper color by multiple absorptions. Ifthe nano-structured film has gaps or defects only on the nano-scale(such gaps or defects being too small to act as resonators for light),the light passes through the film only twice at most (in and out). Incontrast, if the film has “optical gaps”, the light bounces aroundwithin the film and so the film has a much greater absorptionefficiency.

FIG. 12 is a microscopic view of a nano-structured titania film(generally designated 1200) modified with optical gaps 1202, showingregions 1, 2, and 3 as openings that allow for enhanced contrast.

For ease of illustration, FIG. 12 shows the optical gaps as extendingcompletely through the modified film . However, the same optical effectis produced by gaps in the form of voids within the film thickness, andin practice this type of void gap is generally preferred since suchvoids are readily produced by including a sacrificial spacer particle inthe layer and then removing the particle, for example by thermaldecomposition or dissolution in a solvent.

The nano-structured titania film 1200 provided with optical gaps 1202may be prepared as follows:

Step 1: Preparing Titania Containing with Sacrificial Spacer Particles.

In this step, a colloidal titania dispersion is prepared by hydrolysisof titanium tetraisopropoxide (titanium tetrachloride couldalternatively be used). The average diameter (7 nm) of the initiallyformed crystallites is increased to 12 nm by autoclaving at 200° C. for12 hours. Concentrating the autoclaved dispersion to 160 g/l and addingCarbowax20000 (40% wt. equiv. of titania) yields a white viscoussolution. The sacrificial spacer particles of polystyrene or latex, inthe order of 0.2 to 0.4 μm in diameter, are added to this white viscoussol.

Step 2: Depositing Titania on Oxide-Coated Conductive Layer

In this step, a 4 μm thick layer of the above sol is deposited using ascreen-printing technique on to the conductive layer 908 provided on theglass substrate 906. The resulting gel-film is dried in air for 1 hour,then sintered at 450° C. for 12 hours. At a temperature as high as 450°C., the sacrificial particles are eliminated (burned off), therebycreating optical gap regions 1, 2, and 3.

There are many other possible ways of creating nano-structured titaniafilms with gaps 1202. One possible process is to make the sol from whichthe titania particles are deposited more dilute, such that as thetitania particles are deposited (as in step 2 above), there are opticalgaps between the titania regions. Another means to produce gaps is toproduce a microcell structure on oxide-coated conductive layer 908.

Section B3: Third Embodiment—Enhancing the Contrast of the RoughenedSurface by Backing

Another means to enhance the contrast of a nano-structured titania filmis by creating a backing behind the nano-structured film, since the filmitself is somewhat transmissive in thinner regions and less transmissivein thicker regions. The idea is to include a bright, scattering layer inthe device so that there is a large contrast between the on and offstates. This scattering layer, similar to a white paint layer, can beincluded wherever useful, but is preferably provided on the rear surfaceof the nano-structured titania layer, i.e., on the surface of thistitania layer facing the electrolyte. This enables light passing throughthe nano-structured titania layer to be reflected back through thisnano-structure layer with passing through the electrolyte, thusimproving the contrast of the display. The scattering layer could be apolymer/particle mixture. typically cast from solution, for example asolution in an organic solvent.

FIG. 13 shows an electrochromic display (generally designated 1300)which closely resembles the display 1000 shown in FIG. 10, and, asindicated by the corresponding reference numerals in the two Figures.However, the display 1300 lacks the titania particles 1022 dispersed inthe electrolyte 1020 of display 1000, but is instead provided with areflective titania layer 1302 on the rear surface of the nano-structuredtitania layer 1010. The reflective titania layer 1302 serves to ensurethat, if any light passes through transmissive regions ofnano-structured titania film 1010, the reflective titania layer 1302reflects back this light, thus enhancing the contrast between theactivated region B and non-activated region A. (Note that although, forease of illustration, the layer 1302 is shown in FIG. 13 as a layer ofuniform thickness, in practice this layer would normally be depositedupon, and would tend to planarize, the underlying nano-structured layer1010.)

Section B4: Fourth Embodiment—Low-Cost Production Method withoutSintering

In order to reduce the cost of making an electrochromic display, it isimportant to reduce or eliminate the high temperatures and/or timenecessary to perform the fusing step, which in the aforementioned U.S.Pat. No. 6,301,038, WO 01/27690, and Wood article, is a 450° C., 12-hourprocess. An additional benefit to significantly reducing the temperatureof the process is that flexible polymer substrates can be used into thelower temperature process.

One method to produce a very low-temperature and high-contrastelectrochromic display is to use silica-coated titania particles, whichmay be prepared by the following process, prior to the coating of thetitania particles on the substrate:

Step 1:—Add silica-coated or non-coated titania to water such that theresulting solution contains at least about 20 percent of the titania.

Step 2:—Add base-stabilized sodium silicate (known commercially as“water glass”; this material is stabilized with sodium hydroxide) to thesolution from Step 1.

Step 3:—Add a wax (for example, Carbowax) to the solution from Step 2 tocreate a white viscous gel.

Step 4:—Lower the pH of the solution to about 9-10 using an acid such assulfuric acid.

Step 5:—Deposit the solution prepared in Step 4 on to the glasssubstrate.

Step 6:—Heat and dry the resultant film from Step 5 at 95° C. for threehours to fuse the silica-coated titania and remove the wax (heatingabove 100° C. may be desirable to remove all moisture).

This method may be modified by adding ethanol to the reaction mixture inStep 1, replacing Step 2 by adding tetraethoxysilane to the mixtureformed in Step 1, carrying out Step 3 in the same manner as before,replacing Step 4 with a step of adding ammonium hydroxide to raise thepH of the solution to about 9-10, and carrying out Steps 5 and 6 in thesame manner as before. See the aforementioned 2002/0185378.

The resulting film is highly porous and comprises a large amount of voidspace to allow for a high deposition concentration of the chromophore.The final product is a nano-crystalline titania coated and linked by adense, amorphous layer of silica.

Another method to apply the silica-coated titania particles to thesubstrate is through liquid phase sintering with pressure, whereby theviscous sol created as described above is deposited using a roll-to-rolldrum process, with the pressure of the drums creating the heat to bondthe silica-coated titania particles to the conducting glass substrate.

A further method of coating the nano-structured titania film is todeposit alumina by sputter deposition after a 24-hour fusing cure (1000Å). Alumina can also be deposited from solution using aluminum sulfateand sulfuric acid in dilute aqueous solution. The aluminum sulfate canbe admixed with sodium silicate to permit co-deposition of alumina andsilica.

A further method of coating the nano-structured titania film is tosputter, to deposit using low-temperature chemical vapor deposition, orto spin apply (SOG or “spin on glass”) silica on the titania to acoating thickness of about 1000 Å (100 nm).

FIG. 14A shows a roll-to-roll drum process using of a first drum 1402, asecond drum 1404, and a web 1406 processing a slurry 1408 of the viscoustitania sol created as described above.

FIG. 14B, which is an enlarged view of the area within circle B in FIG.14A, shows a detailed view of the web 1406, which consists of a firstglass substrate 1410, and a conductive layer 1412.

FIG. 14C, which is an enlarged view of the area within circle C in FIG.14A, shows a detailed view of the coated web, which carries a pluralityof silica-coated titania particles 1414. These silica-coated titaniaparticles 1414 on conductive layer 1412 are formed by applying pressureusing the first drum 1402 and the second drum 1404.

It will be appreciated by those skilled in electro-optic displaytechnology that any of the embodiments of the second aspect of thepresent invention can be used singly or together, and may also, asalready mentioned, by combined with the other aspects of the presentinvention. Thus, titania in the electrolyte solution can be used withany type of nano-crystalline substrate. Any nano-crystalline substratecan have optical gaps, and any gap structure can have a thick titaniabacking In addition, any of the means to produce low-cost, flexibleintegrated displays can be used in combination with the enhancedcontrast displays.

Displays in accordance with the second aspect of the present inventionmay make use of any of the electrolytes (and other materials, processesand techniques) described in the aforementioned U.S. Pat. No. 6,301,038,WO 01/27690 and Wood article. Thus, preferred electrolytes comprise atleast one electrochemically inert salt optionally in molten form insolution in a solvent. Examples of suitable salts includehexafluorophosphate, bis-trifluoromethanesulfonate,bis-trifluoromethylsulfonylamidure, tetraalkylammonium,dialkyl-1,3-imidazolium and lithium perchlorate.

Examples of suitable molten salts include trifluoromethanesulfonates andbis-trifluoromethylsulfonylamidures. 1-Propyl-dimethyl imidazoliumbis-trifluoro and lithium perchlorate are particularly preferred.

Section C: Display with Reduced Susceptibility to Environmental Factors

As already mentioned, a third aspect of the present invention relates toreducing the susceptibility of electrochromic displays to environmentalfactors. Among such factors against which it is desirable to protectelectrochromic displays are various types of electromagnetic radiation,especially ultraviolet radiation, moisture, oxygen (typically, ofcourse, atmospheric oxygen) and high electrostatic fields, both externaland internal. Obviously, any given electrochromic display may requireprotection against more than one of these factors, depending upon thespecific materials used in the display and the environment in which thedisplay is being used; for example, a display used within a retail storemay not be exposed to large amounts of ultra-violet radiation and hencemay not need elaborate protection against such radiation, whereas adisplay exposed to full sunlight may need much more elaborateprotection. The various ways in which electrochromic displays may beprotected will be largely described separately below, but methods forcombining the various embodiments of the third aspect of the presentinvention will readily be apparent to those skilled in the art ofelectro-optic display construction.

Section C1: First Embodiment—Ultra-Violet Radiation Seal

A first embodiment of the third aspect of the present invention providesa structure for and method of protecting electrochromic displays againstenvironmental factors by covering the glass substrates of such displayswith an ultra-violet filter and a layer of plastic. These additionallayers eliminate the light degradation of the redox chromophore whichare apparently caused by photocatalytic reactions at the titania film.

FIG. 15 shows an electrochromic display (generally designated 1500)comprising a first glass substrate 1506, a first conductive layer 1508,a nano-structured titania film 1510, a redox chromophore 1512, anelectrolyte solution 1520, a plurality of conductive elements 1516, anda second glass substrate 1514, all of which are similar to thecorresponding integers previously described with reference to FIGS. 9and 13. However, the display 1500 further comprises an electrolyte seal1530, a top UV filter 1532, a bottom UV filter 1534, a top plastic layer1536, a bottom plastic layer 1538, a plurality of terminal pads 1540,and a plurality of openings 1542. (The terms “top” and “bottom” will beused below solely with reference to the orientation of the display 1500shown in FIG. 15. As previously explained, electrochromic displays suchas the display 1500 are intended to be viewed from the surface carryingthe nano-structured titania film, i.e., from below in FIG. 15, so thatthe “top” surface referred to below might be more accurately describedas the “non-viewed” surface of the display and the “bottom” surface asthe viewed surface.)

The central section (denoted “A” in FIG. 15) of display 1500 isessentially identical to the corresponding part of the display 400 shownin FIG. 4 and can be formed by the method of FIG. 2.

Top UV filter 1532 and bottom UV filter 1534 are layers of a polymerdoped with a commercial UV absorber (for example, Tinuvin—RegisteredTrade Mark); such absorber doped polymer layers are availablecommercially. After coating top UV filter 1532 and bottom UV filter 1534with thin sheets of top plastic layer 1536 and bottom plastic layer1538, openings 1542 are formed where connections to terminal pads 1540are needed. In some cases where one surface of the display is notexposed to UV radiation (for example, because that surface is mountedwithin an opaque enclosed, the UV filter and plastic layer may beomitted; for example, the UV filter 1532 and plastic layer 1536 might beomitted from the display 1500 if the non-viewed surface of the displaywere not exposed to UV radiation.

During manufacture, the central section “A” is kept in an environmentsubstantially free from UV so as to not expose the titania layer to UVradiation (i.e., display processing after fusing of the titania film isdone in a red or yellow light environment). A preferred method of makingthe electrochromic display 1500 is as follows:

Step 1: Creating UV Filter Material

As already mentioned, the necessary absorber-doped polymer materials areavailable commercially, or a mixture of the absorber and polymer insolution may be prepared by conventional techniques well known in theart.

Step 2: Applying UV Filter Material to the Top Surface (Optional)

If a solution is being applied, the UV filter material created in Step 1is spread using for example spray coating, bar coating or spin coatingto a final thickness of about 2 μm on second glass substrate 1514 toform top UV filter 1532. If a preformed absorber-doped polymer layer isused, this layer is simply laminated in position, using an adhesive,such a polyester adhesive, if necessary.

Step 3: Applying UV Filter Material to Bottom Surface

If a solution is being applied, the UV filter material created in Step 1is spread using the same techniques as in Step 2 to a final thickness ofabout 2 μm on first glass substrate 1506 to form bottom UV filter 1534.If a preformed absorber-doped polymer layer is used, this layer issimply laminated in position, using an adhesive, such a polyesteradhesive, if necessary.

Step 4: Applying Protective Plastic Layer to the Top UV Filter Material(Optional)

A thin (2 mm) clear plastic layer is applied (by lamination orroll-pressing) on top UV filter 1532 to form top plastic layer 1536.

Step 5: Applying Protective Plastic Layer to the Bottom UV FilterMaterial

A thin (2 mm) clear plastic layer is applied (by lamination orroll-pressing) on bottom UV filter 1534 to form bottom plastic layer1538.

Step 6: Forming Openings Through Top Plastic Layer and Top UV FilterLayer (Where Needed)

Portions of top plastic layer 1536 and top UV filter 1532 may be removed(by wet etching or other means) to form openings 1542 where connectionsto terminal pads 1540 are needed.

Step 7: Forming Openings Through Bottom Plastic Layer and Bottom UVFilter Layer

Portions of bottom plastic layer 1538 and bottom UV filter 1534 areremoved (by wet etching or other means) to form openings 1542 whereconnections to terminal pads 1540 are needed. It should, however, benoted that if, as is commonly the case, the conductive layer 1508 is acommon electrode extending across the whole display, the necessaryconnection to this common electrode may conveniently be made at one edgeof the display, thereby avoiding any undesirable optical effects due tothe formation of openings in the viewed surface of the display 1500.

UV filters 1532 and 1534 shield the electrochromic display 1500 from UVradiation, for example the UV radiation found in sunlight.

The UV filters 1532 and 1534 may, for example, be formed from ethylenevinyl acetate (EVA) or from a polycarbonate (for example, the materialknown as Lexan (Registered Trade Mark), produced by General Electric) ofa thickness of, for example, 1 to 5 mils (25 to 127 μm). A polymethylmethacrylate (PMMA) based layer, typically having a thickness of about 1to 10 mil (25 to 254 μm) may also be used.

Section C2: Second Embodiment—Moisture Seal

The third aspect of the present invention also provides a structure andmethod for sealing an electrochromic display from moisture, and apreferred structure for this purpose is shown in FIGS. 16 and 17.

FIG. 16 is a cross-section through an electrochromic display (generallydesignated 1600) comprising a first glass substrate 1606, a conductivelayer 1608, a nano-structured titania film 1610, a redox chromophore1612, an electrolyte solution 1620, conductive elements 1616, a secondglass substrate 1614, an electrolyte seal 1630, and terminal pads 1640,all of which are similar to the corresponding integers of display 1500described above with reference to FIG. 15, and all of which are includedwithin a central section “A”, which is shown in FIG. 16 bounded by adotted line. However, the display 1600 also comprises a moisture seal1640.

The central section “A” of the electrochromic display 1600 may beprepared by methods known in the art and already described, for exampleis method 200 shown in FIG. 2.

Moisture seal 1640 is approximately 2.5 mm in width and equal inthickness to electrolyte seal 1630. The two seals are spacedapproximately 2.5 mm apart. Electrolyte seal 1630 is a deposited epoxyresin as described in the prior art and above with reference to FIGS. 3Aand 3B.

FIG. 17 is a top plan view, similar to that of FIG. 3B, of part of anassembly used to produce display 1600; FIG. 17 shows the relativepositions of the electrolyte seal 1630 and the moisture seal 1640. Itwill be seen that electrolyte seal 1630 is provided with an electrolyteseal gap 1632, while moisture seal 1640 is provided with a moisture sealgap 1642. These gaps 1632 and 1642 are provided for the same purpose asthe opening 118 shown in FIG. 3B, namely to allow the display 1600 to befilled with electrolyte solution 1620 through the gaps 1632 and 1642.

Moisture seal 1640 is a hydrophobic seal formed from a glass, polymer,or metal. If moisture seal 1640 is made of glass, liquid glass may beapplied into the gap between the glass substrates 1606 and 1614 andallowed to cure. The moisture seal 1640 may be formed from a hydrophobicpolymer, for example polyethylene, polytetrafluoroethylene, polystyrene,polyvinyl chloride, polyethylene terephthalate, polyisoprene,polypropylene or a polysiloxane. A calcium-based material or otherdesiccant may be included within the display inside the moisture seal1640 to scavenge any moisture which ultimately manages to penetrate theseal. Appropriate calcium-based materials and desiccants include calciumcarbonate, magnesium sulfate, sodium sulfate, calcium oxide and alumina.

The display 1600 may be manufactured by a modification of the process200 shown in FIG. 2. The moisture seal 1640 may be deposited on thefirst glass substrate 1606 before, simultaneously with, or after theelectrolyte seal 1630, leaving the gap 1642 in seal 1640; see step 210of method 200. The electrolyte solution 1620 is then filled under vacuumthrough moisture seal gap 1642 and electrolyte seal gap 1632 in the sameway as in step 212 of method 200, and the electrolyte seal gap 1632 issealed, as described above with reference to step 214 of method 200,through moisture seal gap 1642, leaving moisture seal gap 1642 open.Finally, moisture seal gap 1642 is sealed using a moisture seal materialand a syringe-type applicator.

As already noted, the electrochromic display 1600 may include acalcium-based material or other desiccant as a patch inside the moistureseal 1640 to act as a scavenger to remove water from the electrolytesolution 1620. Although elemental calcium could be used for thispurpose, its use is not generally recommended since the elemental formtends to be pyrophoric.

Section C3: Third Embodiment—Oxygen and Moisture Seal

A third embodiment of the third aspect of the present invention providesa structure for and method of sealing an electrochromic display fromboth oxygen and moisture. An electrochromic display sealed againstmoisture has been described above. A similar structure may be used toseal against both oxygen and moisture, except that the oxygen seal is athird sealing layer, along with the moisture and electrolyte seals.

The three seals (electrolyte, moisture and oxygen) can be in any order,and in some case a single material may serve as a seal against any twoor all three materials.

Section C4: Fourth Embodiment—Coated Titania

Reference has already been made above, during the discussion of thesecond aspect of the present invention, to the use of nano-structuredtitania films in which the titania is coated with silica and/or alumina.Although the purpose for such coating of the titania in the secondaspect of the present invention is to reduce the sintering temperatureof the titania, such coating of the titania has the additional advantageof eliminating, or at least reducing, the photoactive titania surfaceand replacing it with a substantially non-photoactive silica or aluminasurface (the surface could of course comprise a mixture of these twomaterials). This coating thus reduces or eliminates the degradation ofthe redox chromophore from ultra-violet radiation due to thephotocatalytic reaction of the nano-structured titania film.

The coated titania may be prepared by any of the methods described inSection B4 above.

Section C5: Fifth Embodiment—Electrostatic Shield

A fifth embodiment of the third aspect of the present invention providesprotection of an electrochromic display against electrostatic currents.The electrostatic protection means is formed from a bi-directional diodeor unidirectional diode with controlled reverse breakdowncharacteristics. One prior art method, described in U.S. Pat. No.5,930,607, “Method to prevent static destruction of an active elementcomprised in a liquid crystal display device,” uses an electrostaticprotection element composed of a MOS transistor connected between theelectrode for connecting the external terminal, and the joint electricpotential line. Each of the drive lines connected to terminal pads aretied to ground through the bi-directional diode or unidirectional diode.

Section C6: Sixth Embodiment—Glass Posts or Beads

A sixth embodiment of the third aspect of the present invention relatesto reducing cell-to-cell field effect and eliminating high fields bydeveloping glass posts on the glass substrates.

FIG. 18 is a schematic section, generally similar to that of FIG. 16,though an electrochromic display (generally designated 1800) comprisinga first glass substrate 1806, a conductive layer 1808, a nano-structuredtitania film 1810, a redox chromophore 1812, an electrolyte solution1820, conductive elements 1816, a second glass substrate 1814, anelectrolyte seal 1830, and terminal pads 1840, all of which aresubstantially identical to the corresponding integers shown in FIG. 16.However, the electrochromic display 1800 further comprises a pluralityof glass posts 1850, only one of which is shown in FIG. 18. The glassposts 1850 are glued to the second glass substrate 1814, and areprovided in regions that enclose each cell (as defined by the conductiveelements 1816). The glass posts 1850 can alternatively be etched fromthe second glass substrate 1814 if the initial second glass substrate1814 is made thicker. The formation of the posts 1850 is done before anyof the prior art processes are used to form the second glass substrate1814.

An alternative method of reducing cell-to-cell field effect is todisperse glass beads into electrolyte solution 1820 to prevent theelectrochromic display 1800 from collapsing. The “beads” used need notbe spherical and may, for example, have any of the forms described inU.S. Pat. No. 6,392,786.

The present invention may make use of any of the electrolytes mentionedin the aforementioned U.S. Pat. No. 6,301,038, WO 01/27690 and Woodarticle, as already discussed above.

Section D: Integrating and Controlling an Electro-Optic Display

As already mentioned, the fourth aspect of the present invention relatesto systems and methods of operating an electro-optic display. Incontrast to the first three aspects of the present invention, which areconfined to electrochromic displays, the fourth aspect is not confinedto such displays, but can be applied to any of the types ofelectro-optic displays discussed in the introductory part of thisapplication.

However, perhaps the most complex type of electro-optic display to driveis an electrochromic display, such as the electrochromic displays basedupon nano-structured titania films previously described. Suchelectrochromic displays are affected by factors such as ion transportefficiency and variable diodic behavior of the nano-structured titaniafilm, which are not present in other types of electro-optic displays.Accordingly, hereinafter a preferred embodiment of the invention forcontrolling an electrochromic display will first be described, andthereafter application of the same general principles to other types ofelectro-optic displays will be discussed.

Thus, the fourth aspect of this invention provides a current drive meansto drive an electrochromic cell of a display comprising multiple cells.Because of the electrical model of the electro-chromophore cell, eachcell must be driven based upon the behaviors of at least the: (1)steady-state response of the cell; (2) time-varying response of thecell; (3) interaction with adjacent cells; (4) light reflectivity versusion transport curve for the redox chromophore of the cell; (5) iontransport efficiency; (6) variable diodic behavior of thenano-structured titania film of the cell; (7) interaction of opticalfeedback of the cell; (8) electrolyte potential changeover of the cell;(9) changes over operating life of the electrolyte solution performanceof the cell; (10) changes over operating life of the redox chromophoreof the cell; (11) effects of ambient and operating temperatures on cellperformance; and (12) effects of ambient light on display quality. Inorder to include all these behaviors, a means to drive each cell basedupon an algorithm for each of these behaviors is required. Many of thesebehaviors are dependent on environmental factors, such as ambient light,electrolyte potential, temperature, etc. Because of this dependency,these environmental factors must be monitored and the algorithms need tobe adjusted by these monitored results. Thus, the basic system not onlyincludes the means to direct drive each cell by a current driver, butalso a processor for translating the required image input data to drivethe display. The processor may use algorithms stored in memory andsensor data sensed from a sensor to dynamically drive each of thecurrent drives for each cell.

FIG. 19 shows a block diagram of an electrochromic display system(generally designated 1900) consisting of a plurality ofnano-crystalline electrochromic display cells 1902 in a display area1904, a plurality of current mode drivers (CMD's) 1906, a plurality ofdrive currents 1908, a memory 1910, a processing unit 1912, adata/address bus 1914, a device interface 1916, a plurality ofanalog-to-digital converters (A/D's) 1918, an electrochromophore sensor1920, an electrolyte sensor 1922, a temperature sensor 1924, and a lightsensor 1926.

Processing unit 1912 is a semiconductor device, such as a microprocessoror micro controller, well known in the art. It is electrically connectedto current mode drivers 1906, memory 1910, and A/D's 1918 viadata/address bus 1914. Communication of address, data, and commandinformation is transferred using standard digital electronic interfacetechniques. Processing unit 1912 is also electrically connected todevice interface 1916. Device interface 1916 provides the electrical andprotocol interface between electrochromic display system 1900 and theelectronic device it services, such as a computer, cell phone, orpersonal digital assistant (PDA).

A/D's 1918 are semiconductor analog-to-digital converters of a typeknown in the art, and are electrically connected to electrochromophoresensor 1920, electrolyte sensor 1922, temperature sensor 1924, and lightsensor 1926.

Current mode drivers 1906 are electrically connected to nano-crystallineelectrochromic display cells 1902 so that one CMD 1906 is connected toone and only one nano-crystalline electrochromic display cell 1902.CMD's 1906 source or sink drive currents 1908. (It will be appreciatedby those skilled in the art of driving displays that the arrangementshown in FIG. 19 can readily be adapted for active matrix driving of anelectrochromic display. In such an active matrix display, the currentmode drivers are connected to source lines and select drivers areconnected to select lines, and by sequential activation of the selectlines, voltages or currents can be applied to the appropriate pixelsfrom the source lines.)

Memory 1910 is a non-volatile semiconductor memory of a type known inthe art (ROM, EPROM, EEPROM, NVRAM) and sized according to theapplication requirement. Both data and program executable images residein memory 1910.

In operation, electrochromic display system 1900 functions to interpretcommands from the electronic device it is servicing, to determine thedesired display, to assess the state of the display, to applyoptimization rules based on that state, and to drive eachnano-crystalline electrochromic display cell 1902 with the appropriateelectrical current to effect the new display image.

Basic Process

FIG. 20 shows a flow chart of a method (generally designated 2000) fordriving electrochromic display system 1900, this method 2000 includingthe following steps:

Step 2010: Receiving Display Data from Device

In this step, electrochromic display system 1900 receives digital codedelectronic data from the electronic device it is servicing via deviceinterface 1916. The received data set defines the next required displayimage on a cell-by-cell basis, as well as the state of each cell(On/Off/Certain Grey or color scale). Device interface 1916 performs thealgorithms to translate the display image to cell-by-cell data beforethis data is transferred to processing unit 1912.

Step 2020: Interpreting Display Data

In this step, device interface 1916 performs a hard-coded algorithm totranslate the data set received from the serviced device into the nativedata format of processing unit 1912.

Step 2030: Creating Virtual Cell Map in Memory

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910,interprets the data received from device interface 1916 and creates avirtual cell map in memory 1910. This map is a row/column matrix withthe desired next state for each nano-crystalline electrochromic displaycell 1902 in electrochromic display system 1900.

Step 2040: Assessing State of Display

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, andassesses the state of electrochromic display system 1900. Thisassessment includes the previously driven state of each nano-crystallineelectrochromic display cell 1902 and several environmental variables.This step is further detailed in FIG. 21.

Step 2050: Applying State-Driven and Static Rules to Each Cell

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, andapplies both state-driven and static rules for determining whatelectrical current to apply to each nano-crystalline electrochromicdisplay cell 1902 to achieve the desired display image. The resultingcurrent level values to be applied are stored in memory 1910. This stepis further detailed in FIG. 22.

Step 2060: Establishing Current Mode Driver for Each Cell

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, andtranslates the current level values to be applied to eachnano-crystalline electrochromic display cell 1902 into digital data andcontrol signals for interpretation by CMD's 1906.

Step 2070: Setting Current Drive for Each Cell

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, andtransmits data and command signals via data/address bus 1914 to CMD's1906. The CMD's 1906 interpret the data and command string, and sourceor sink the required drive currents 1908.

Step 2080: Recording Cell State in Memory

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, andwrites the new cell states for all nano-crystalline electrochromicdisplay cells 1902 into memory 1910.

Process for Sensing and Adapting

The electrochromic display system 1900 of the present invention isdesigned to adapt to changes in the state of the display in order tooptimize the display properties. The first step in this adaptation isthe accurate determination of the state of various aspects of thedisplay.

FIG. 21 shows a flow chart of a method (generally designated 2100) forassessing the state of the display, including the following steps:

Step 2110: Determining Previous Set State

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, anddetermines the previously set state of each nano-crystallineelectrochromic display cell 1902. The previous set state for each cellis stored in a specified table in memory 1910. Note that during initialpower-up, all cells are be forced to a known state, either colored orbleached.

Step 2120: Determining State of Electrolyte

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, anddetermines the operating state of the electrolyte solution. Processingunit 1912 transmits data and command signals via data/address bus 1914to A/D's 1918, which interpret the data stream, select the input channelelectrically connected to electrolyte sensor 1922, sample the analogsignal present on the input channel, and convert the magnitude of thesignal to the digital domain. Upon completion of the conversion, A/D's1918 place the resulting digital data on data/address bus 1914.Processing unit 1912 reads the data from A/D's 1918 and stores this datain memory 1910.

Step 2130: Determining State of Electrochromophore

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, anddetermines the operating state of the redox chromophore. Processing unit1912 transmits data and command signals via data/address bus 1914 toA/D's 1918, which interpret the data stream, select the input channelelectrically connected to electrochromophore sensor 1920, sample theanalog signal present on the input channel, and convert the magnitude ofthe signal to the digital domain. Upon completion of the conversion,A/D's 1918 place the resulting digital data on data/address bus 1914.Processing unit 1912 reads the data from A/D's 1918 and stores it inmemory 1910.

Step 2140: Determining Display Temperature

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, anddetermines the display temperature. Processing unit 1912 transmits dataand command signals via data/address bus 1914 to A/D's 1918, whichinterpret the data stream, select the input channel electricallyconnected to temperature sensor 1924, sample the analog signal presenton the input channel, and convert the magnitude of the signal to thedigital domain. Upon completion of the conversion, A/D's 1918 place theresulting digital data on data/address bus 1914. Processing unit 1912reads the data from A/D's 1918 and stores it in memory 1910.

Step 2150: Determining Ambient Light Level

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, anddetermines the ambient light level. Processing unit 1912 transmits dataand command signals via data/address bus 1914 to A/D's 1918, whichinterpret the data stream, select the input channel electricallyconnected to light sensor 1926, sample the analog signal present on theinput channel, and convert the magnitude of the signal to the digitaldomain. Upon completion of the conversion, A/D's 1918 place theresulting digital data on data/address bus 1914. Processing unit 1912reads the data from A/D's 1918 and stores it in memory 1910.

Process for Modifying Display Based Upon Changes Sensed

Once the state of the display has been thus determined, a series ofalgorithms are applied to the cell state definitions in the previouslycreated virtual cell map in memory. Each rule modifies the driverequirements necessary for accurately displaying the image. FIG. 22 is aflow chart of a method (generally designated 2200) for applying thesealgorithms. The flow chart shown in FIG. 22 is for a simple case whereinteraction between different display parameters is not directly takeninto account. However, those skilled in the art will realize that a morecomplex multiple interaction state-based flow can be readily achievedwith modern programmatic control. Method 2200 includes the followingsteps:

Step 2210: Applying Steady-State Circuit Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesthe previously determined previous driven cell state, a look-up tablewith values for steady-state behavior of electrical circuit model 500(FIG. 5), and state change conversion models for black and white (seethe Table below) to algorithmically determine the first order of drivecurrent for each nano-crystalline electrochromic display cell 1902 indisplay 1900. The look-up table for steady-state behavior of electricalcircuit model 500 may be determined for a class of displays or during aone-time calibration during manufacturing.

TABLE Black/white data Final State Previous State Colored BleachedColored 0 +1 Bleached −1 0

Step 2220: Applying Dynamic Circuit Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for dynamic behavior of electrical circuitmodel 500 to determine the dynamic compensation of drive current 1908for each nano-crystalline electrochromic display cell 1902. The look-uptable for the dynamic behavior of electrical circuit model 500 may bedetermined for a class of displays or during a one-time calibrationduring manufacturing.

Step 2230: Applying Adjacent Cell Interaction Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for adjacent cell interference behavior ofelectrical circuit model 500 to determine the adjacent cell interferencecompensation of drive current for each nano-crystalline electrochromicdisplay cell 1902. The look-up table for the adjacent cell interferencebehavior of electrical circuit model 500 may be determined for a classof displays or during a one-time calibration during manufacturing.

Step 2240: Applying Ion Transfer Efficiency Compensation Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for ion transfer efficiency behavior ofnano-crystalline electrochromic display cells 1912 to determine theadjacent cell interference compensation of drive current for each cell.The look-up table for the ion transfer efficiency behavior ofnano-crystalline electrochromic display cells 1902 may be determined fora class of displays or during a one-time calibration duringmanufacturing.

Step 2250: Applying Titania Diodic Behavior Compensation Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for the diodic behavior of nano-structuredtitania film 430 (FIG. 4) to determine the compensation of drive currentfor each cell. The look-up table for the nano-structured titania film430 behavior may be determined for a class of displays or during aone-time calibration during manufacturing.

Step 2260: Applying Optical Feedback Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for the optical feedback of electrochromicdisplay system 1900 to determine the compensation of drive current 1908for each cell 1902. The look-up table for the optical feedback behaviormay be determined for a class of displays, during a one-time calibrationduring manufacturing, or by real-time measurement.

Step 2270: Applying Electrolyte Aging Compensation Rule

In this step, processing unit 1912 executes code stored in memory 1910and uses a look-up table with values for the operating state ofelectrolyte solution 450 (FIG. 4) to determine the compensation of drivecurrent for each cell 1902. The look-up table for the operating state ofelectrolyte solution 450 is created by real-time measurement (step 2120of method 2100, FIG. 21).

Step 2280: Applying Chromophore Aging Compensation Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910 and usesa look-up table with values for the operating state of redox chromophore440 (FIG. 4) to determine the compensation of drive current for eachcell 1902. The look-up table for the operating state of redoxchromophore 440 is created by real-time measurement (step 2130 of method2100, FIG. 21).

Step 2290: Applying Temperature Compensation Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for temperature to determine thecompensation of drive current for each cell 1902. The look-up table fortemperature is created by real-time measurement (step 2140 of method2100, FIG. 21).

Step 2295: Applying Ambient Light Compensation Rule

In this step, processing unit 1912 transmits data and command signalsvia data/address bus 1914, executes code stored in memory 1910, and usesa look-up table with values for ambient light to determine thecompensation of drive current for each cell 1902. The look-up table forthe ambient light is created by real-time measurement (step 2150 ofmethod 2100, FIG. 21).

The steps in methods 2100 and 2200 are not essential to the fundamentalmethod of driving electrochromic display system 1900, and any of thesesteps can be added or eliminated.

Those skilled in the art of driving electro-optic displays willappreciate that although current mode drivers 1906 of FIG. 19 aredescribed as a means to drive each nano-crystalline electrochromicdisplay cell 1902, a voltage driver could be used instead. In this case,the hard-coded algorithms in memory 1910, as well as the means to senseand adapt data for each cell, would be much more complicated because ofthe nature of the electrolytic-based cell. The cell has electricalcircuit model 500 of FIG. 5 with a capacitor, which in a small signalsense has time sensitive and previous state sensitive capacitance. Thus,the algorithms to drive a voltage drive system would be far morecomplex.

Non-Electrochromic Media and Displays

As will be apparent to those skilled in technology of electro-opticdisplays, many of the techniques described above for controllingelectrochromic media and displays, such as the provisions of temperaturesensors and timers, can readily be applied to other types ofelectro-optic displays.

For example, in an encapsulated electrophoretic display, changes in theperformance of the encapsulated electrophoretic medium caused by changesin environmental parameters or aging could be compensated by changingthe drive pulse length. Under any given set of environmental conditions,the waveform can be optimized to provide a balance between over/underdriving and self-erasing. The adaptation can be done by monitoring anyappropriate parameter of the electrophoretic medium or its environmentand using a predetermined formula or look up table to change thewaveforms of the drive pulses. The same type of monitoring could also beused to provide an output signal which could serve as an input to acircuit or algorithm to determine the proper drive waveform, or thesemethods could be used in combination.

For example, as already indicated it is known from laboratoryexperiments that the optimum impulse (which for present purposes isdefined as the integral of the applied voltage with respect to time,although the integral of current through the electrophoretic medium withrespect to time could also be used) to be applied to an encapsulatedelectrophoretic medium is a function of temperature. A thermistor,digital thermometer or other temperature sensor could be used to measurethe temperature of electrophoretic medium, or the environmentimmediately adjacent thereto, and the impulse could be adjusted throughan analogue or digital feedback circuit. A humidity sensor could also beused in cases where the performance of the electrophoretic medium wassubstantially affected by changes in humidity.

Also, the current transient for any convenient unit of the display (forexample, a single pixel, a row or column of a matrix display or thewhole display) resulting from a single switching operation could bemeasured and used to control the waveform. Alternatively, the currentpassing through the electrophoretic medium in some reference state (forexample, during a refresh pulse used to counteract drift of the opticalstate of the display from its original value) could be measured, therebyproviding information about the movement of the electrophoreticparticles within the electrophoretic medium and/or about the capsulewall, binder or other components of the electrophoretic medium.Computations based upon the current/time curve could then be used tocalculate the appropriate drive waveform. A combination of one or moreof the foregoing approaches could be used to vary the drive waveform andcompensate for the effects of temperature upon the electrophoreticmedium; such changes include, but are not limited to viscosity changesof the suspending fluid in which the electrophoretic particles aresuspended.

Since temperature is probably the most important environmental variableaffecting most encapsulated electrophoretic displays, temperaturecompensation of such displays will now be considered in more detail, itbeing understood that the temperature compensation techniques describedbelow may be useful with electro-optic media other than encapsulatedelectrophoretic media.

The two simplest techniques for adjustment of drive waveforms inresponse to variations in temperature are temperature-dependent voltagevariation and temperature-dependent duration variation. These methodscan be used separately or in combination to improve encapsulatedelectrophoretic medium performance. In temperature-dependent voltagevariation, an electronic circuit is required to vary the voltage outputbased on temperature of the environment. The voltage should varyinversely with temperature (i.e. as temperature goes up, voltage outputgoes down) for use with an encapsulated electrophoretic display.Similarly, in temperature-dependent duration variation, an electroniccircuit is required to vary the drive pulse duration based ontemperature of the environment. The duration should vary inversely withtemperature (i.e. as temperature goes up, the drive pulse duration goesdown) for use with an encapsulated electrophoretic display.

FIGS. 23 and 24 of the accompanying drawings illustrate circuits used ina preferred embodiment of a temperature-compensated voltage supply fordriving encapsulated electrophoretic displays; FIG. 23 shows a DC-DCvoltage converter circuit (“voltage booster”) while FIG. 24 shows avoltage feedback circuit. The DC-DC converter circuit in FIG. 23 chargesup the capacitor shown therein to a target voltage while being monitoredby the voltage feedback circuit of FIG. 24. The voltage feedback circuitof FIG. 24 checks this target voltage by comparing a monitor voltage,determined by the resistor voltage divider, to a reference voltage,determined by the voltage divider circuit consisting of a resistor and adiode. When the monitor voltage exceeds the reference voltage, thetarget voltage is reached and the PWM feed to the voltage boostercircuit of FIG. 23 is turned off. When the monitor voltage dips belowthe reference voltage, the voltage booster is turned back on to raisethe monitor voltage again.

The diode/resistor voltage divider shown in FIG. 24 is the crucial partof this temperature-compensated voltage supply. The diode has aresistivity characteristic that is temperature-dependent, i.e., itsresistivity varies inversely with temperature. In other words, the diodebecomes more conductive as temperature increases. In the resistordivider circuit, this temperature variation of the diode in turn causesthe reference voltage to fall as temperature increases. The voltagebooster circuit of FIG. 23 will accordingly be turned off at lowertarget voltages, effectively producing lower high voltage outputs athigher temperatures.

As already indicated, an encapsulated electrophoretic medium tends todegrade during use, and the rate of such degradation increases with theamount of electrical current passed through the medium. At highertemperatures, the medium becomes more conductive and therefore would seemore current if the drive pulse voltage and duration were maintainedconstant at all temperatures. Reducing the drive pulse voltage withtemperature using the circuitry of FIGS. 23 and 24 reduces the rate ofdegradation of the medium at higher temperatures and thus extends theworking lifetime of the medium.

The circuitry shown in FIGS. 23 and 24 also lowers the variation in theoptical response of an encapsulated electrophoretic medium withvariation in temperature. The circuitry drives the medium at a highervoltage and/or with a longer voltage pulse at lower temperatures inorder to saturate the medium, while driving the medium at a lowervoltage and/or shorter voltage pulse at higher temperatures in order toprevent over-saturation of the medium. The circuitry also has thepotential to make the switching response time curve of the encapsulatedelectrophoretic medium more uniform despite variations in temperature.

The foregoing advantages are achieved simply by modest modifications ofthe drive circuitry needed to drive the electrophoretic medium to enablethis circuitry to monitor the environmental conditions to which themedium is exposed and adapt the drive waveform used. No modification ofthe medium itself is required, and this is advantageous sincemodification of a relatively complex encapsulated electrophoretic mediumto reduce the variation of its electro-optic properties with temperatureis inherently more difficulty than modification of drive circuitry.

The present invention allows electro-optic media to operate under awider range of environmental conditions than would be possible using afixed drive waveform. By incorporating into the display a timer whichmeasures the period for which the medium has been operated, the presentinvention also allows for “aging” of the electro-optic medium which canhave an undesirable impact on the performance of the medium.

Numerous changes and modifications can be made in the preferredembodiments of the present invention already described without departingfrom the spirit and skill of the invention. Accordingly, the foregoingdescription is to be construed in an illustrative and not in alimitative sense.

1. An electro-optic display comprising: an electro-optic medium; atleast one electrode arranged to apply an electric field to theelectro-optic medium; drive means for supplying a driving pulse to theelectrode; a sensor for measuring at least one parameter affecting thebehavior of the electro-optic medium and for producing an output signalrepresentative of the parameter; and control means for receiving theoutput signal from the sensor and controlling the drive means to varythe driving pulse dependent upon the output signal, wherein said sensorcomprises at least one of: (a) a temperature sensor for sensing thetemperature of, or adjacent to, the electro-optic medium; (b) a humiditysensor for sensing the humidity of, or adjacent to, the electro-opticmedium; and (c) a timer for measuring the operating time of theelectro-optic medium.
 2. An electro-optic display according to claim 1wherein said sensor comprises a temperature sensor for sensing thetemperature of, or adjacent to, the electro-optic medium.
 3. Anelectro-optic display according to claim 1 wherein said sensor comprisesa humidity sensor for sensing the humidity of, or adjacent to, theelectro-optic medium.
 4. An electro-optic display according to claim 1wherein said sensor comprises a timer for measuring the operating timeof the electro-optic medium.
 5. An electro-optic display according toclaim 1 having a plurality of electrodes arranged to apply an electricfield to a plurality of pixels of the electro-optic medium, and whereinthe drive means is arranged to vary the driving pulse to one electrodebased upon the driving pulse applied to at least one other electrode. 6.An electro-optic display according to claim 1 wherein the electro-opticmedium is an electrophoretic medium comprising a suspending fluid, and aplurality of electrically charged particles suspended in the suspendingfluid and capable of moving therethrough on application of an electricfield to the suspending fluid.
 7. An electro-optic display according toclaim 6 wherein the suspending fluid and the plurality of electricallycharged particles are retained within a plurality of capsules.
 8. Anelectro-optic display according to claim 6 wherein the suspending fluidand the plurality of electrically charged particles are present as aplurality of discrete droplets and a continuous phase of polymericmaterial surrounds the droplets.
 9. An electro-optic display accordingto claim 6 wherein the suspending fluid and the plurality ofelectrically charged particles are retained within a plurality ofcavities formed in a carrier medium.
 10. An electro-optic displayaccording to claim 1 wherein the electro-optic medium is a rotatingbichromal member medium or an electrochromic medium.
 11. Anelectrochromic display comprising an electrolyte and anelectrochromically-active layer comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto, the electrolyte having a lightscattering and/or reflective material dispersed therein.
 12. Anelectrochromic display comprising a nano-porous-nano-crystalline filmcomprising a semiconducting metal oxide having an electroactive compoundwhich is either a p-type or n-type redox promoter or p-type or n-typeredox chromophore adsorbed thereon or otherwise attached thereto, thedisplay having a viewing surface through which an observer can view thedisplay, the display also having, on the opposed side of the film fromthe viewing surface, a layer of a light-scattering or reflectivematerial disposed on the film.
 13. An electrochromic display comprisingan electrochromically-active layer comprising anano-porous-nano-crystalline film comprising a semiconducting metaloxide having an electroactive compound which is either a p-type orn-type redox promoter or p-type or n-type redox chromophore adsorbedthereon or otherwise attached thereto, the film being formed from asemiconducting metal oxide coated with at least one of silica andalumina.
 14. A process for forming an electrochromic display accordingto claim 13, the process comprising: coating particles of asemiconducting metal oxide with at least one of silica and alumina; andforming the coated particles into the film at a temperature not greaterthan about 400 C.